[libc++] Implements Unicode grapheme clustering

This implements the Grapheme clustering as required by
P1868R2 width: clarifying units of width and precision in std::format

This was omitted in the

[libc++] Implements Unicode grapheme clustering

This implements the Grapheme clustering as required by
P1868R2 width: clarifying units of width and precision in std::format

This was omitted in the initial patch, but the paper was marked as completed. This really completes the paper.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D126971

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[libc++][format] Improve string formatters

This changes the implementation of the formatter. Instead of inheriting
from a specialized parser all formatters will use the same generic
parser. This red

[libc++][format] Improve string formatters

This changes the implementation of the formatter. Instead of inheriting
from a specialized parser all formatters will use the same generic
parser. This reduces the binary size.

The new parser contains some additional fields only used in the chrono
formatting. Since this doesn't change the size of the parser the fields
are in the generic parser. The parser is designed to fit in 128-bit,
making it cheap to pass by value.

The new format function is a const member function. This isn't required
by the Standard yet, but it will be after LWG-3636 is accepted.
Additionally P2286 adds a formattable concept which requires the member
function to be const qualified in C++23. This paper is likely to be
accepted in the 2022 July plenary.

Depends on D121530

NOTE parts of the code now contains duplicates for the current and new parser.
The intention is to remove the duplication in followup patches. A general
overview of the final code is available in D124620. That review however lacks a
bit of polish.

Most of the new code is based on the same algorithms used in the current code.

The final version of this code reduces the binary size by 17 KB for this example
code
```

int main() {
{
std::string_view sv{"hello world"};
std::format("{}{}|{}{}{}{}{}{}|{}{}{}{}{}{}|{}{}{}|{}{}|{}", true, '*',
(signed char)(42), (short)(42), (int)(42), (long)(42), (long long)(42), (__int128_t)(42),
(unsigned char)(42), (unsigned short)(42), (unsigned int)(42), (unsigned long)(42),
(unsigned long long)(42), (__uint128_t)(42),
(float)(42), (double)(42), (long double)(42),
"hello world", sv,
nullptr);
}
{
std::wstring_view sv{L"hello world"};
std::format(L"{}{}|{}{}{}{}{}{}|{}{}{}{}{}{}|{}{}{}|{}{}|{}", true, L'*',
(signed char)(42), (short)(42), (int)(42), (long)(42), (long long)(42), (__int128_t)(42),
(unsigned char)(42), (unsigned short)(42), (unsigned int)(42), (unsigned long)(42),
(unsigned long long)(42), (__uint128_t)(42),
(float)(42), (double)(42), (long double)(42),
L"hello world", sv,
nullptr);
}
}

```

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D125606

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[libc++][format] Adds formatter<charT[N], charT>.

This formatter isn't in the list of required formatters in

[format.formatter.spec]/2.2
For each charT, the string type specializations
templat

[libc++][format] Adds formatter<charT[N], charT>.

This formatter isn't in the list of required formatters in

[format.formatter.spec]/2.2
For each charT, the string type specializations
template<> struct formatter<charT*, charT>;
template<> struct formatter<const charT*, charT>;
template<size_t N> struct formatter<const charT[N], charT>;
template<class traits, class Allocator>
struct formatter<basic_string<charT, traits, Allocator>, charT>;
template<class traits>
struct formatter<basic_string_view<charT, traits>, charT>;

Since remove_cvref_t<const charT[N]> is charT[N] the formatter is
required by

[format.functions]/25
Preconditions: formatter<remove_cvref_t<Ti>, charT> meets the
BasicFormatter requirements ([formatter.requirements]) for each Ti in
Args.

Depends on D120921

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D121138

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[libc++] Replace _LIBCPP_HAS_NO_CONCEPTS with _LIBCPP_STD_VER > 17. NFCI.

All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBC

[libc++] Replace _LIBCPP_HAS_NO_CONCEPTS with _LIBCPP_STD_VER > 17. NFCI.

All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBCPP_STD_VER > 17`. Similarly in
the tests, remove `// UNSUPPORTED: libcpp-no-concepts`.

Differential Revision: https://reviews.llvm.org/D121528

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[libc++] Revert "Protect users from relying on detail headers" & related changes

This commit reverts 5aaefa51 (and also partly 7f285f48e77 and b6d75682f9,
which were related to the original commit).

[libc++] Revert "Protect users from relying on detail headers" & related changes

This commit reverts 5aaefa51 (and also partly 7f285f48e77 and b6d75682f9,
which were related to the original commit). As landed, 5aaefa51 had
unintended consequences on some downstream bots and didn't have proper
coverage upstream due to a few subtle things. Implementing this is
something we should do in libc++, however we'll first need to address
a few issues listed in https://reviews.llvm.org/D106124#3349710.

Differential Revision: https://reviews.llvm.org/D120683

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[libcxx][modules] protects users from relying on detail headers

libc++ has started splicing standard library headers into much more
fine-grained content for maintainability. It's very likely that ou

[libcxx][modules] protects users from relying on detail headers

libc++ has started splicing standard library headers into much more
fine-grained content for maintainability. It's very likely that outdated
and naive tooling (some of which is outside of LLVM's scope) will
suggest users include things such as <__ranges/access.h> instead of
<ranges>, and Hyrum's law suggests that users will eventually begin to
rely on this without the help of tooling. As such, this commit
intends to protect users from themselves, by making it a hard error for
anyone outside of the standard library to include libc++ detail headers.

Differential Revision: https://reviews.llvm.org/D106124

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[libc++] Move everything related solely to _LIBCPP_ASSERT to its own file

This is the first step towards disentangling the debug mode and assertions
in libc++. This patch doesn't make any functional

[libc++] Move everything related solely to _LIBCPP_ASSERT to its own file

This is the first step towards disentangling the debug mode and assertions
in libc++. This patch doesn't make any functional change: it simply moves
_LIBCPP_ASSERT-related stuff to its own file so as to make it clear that
libc++ assertions and the debug mode are different things. Future patches
will make it possible to enable assertions without enabling the debug
mode.

Differential Revision: https://reviews.llvm.org/D119769

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[libc++][format][nfc] Header cleanup.

Remove the unneeded macro protection, forward declarations, and
includes.

Reviewed By: #libc, Quuxplusone, ldionne, philnik

Differential Revision: https://rev

[libc++][format][nfc] Header cleanup.

Remove the unneeded macro protection, forward declarations, and
includes.

Reviewed By: #libc, Quuxplusone, ldionne, philnik

Differential Revision: https://reviews.llvm.org/D118925

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[libc++] Normalize all our '#pragma GCC system_header', and regression-test.

Now we'll notice if a header forgets to include this magic phrase.

Differential Revision: https://reviews.llvm.org/D1188

[libc++] Normalize all our '#pragma GCC system_header', and regression-test.

Now we'll notice if a header forgets to include this magic phrase.

Differential Revision: https://reviews.llvm.org/D118800

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[libc++][format] Disable default formatter.

[format.formatter.spec]/5 lists the requirements for the default
formatter. The original implementation didn't implement this. This
implements the default

[libc++][format] Disable default formatter.

[format.formatter.spec]/5 lists the requirements for the default
formatter. The original implementation didn't implement this. This
implements the default formatter according to the Standard.

This adds additional test to validate the default formatter is disabled
and the required standard formatters are enabled.

While adding the tests it seems the formatters needed a constraint for the
character types they were valid for.

Implements parts of:
- P0645 Text Formatting

Depends on D115988

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D115989

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[libc++][format][NFC] Remove some unneeded headers.

Reviewed By: #libc, Quuxplusone, Mordante

Differential Revision: https://reviews.llvm.org/D116175

[libcxx][format] Fix how we handle char traits in formatter<string> and formatter<string_view>

Right now we drop the char_traits template argument, which presumes that
string<_CharT, _Traits> and st

[libcxx][format] Fix how we handle char traits in formatter<string> and formatter<string_view>

Right now we drop the char_traits template argument, which presumes that
string<_CharT, _Traits> and string<_CharT> are interchangeable.

Reviewed By: Mordante, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D112017

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[libc++][format] Adds string formatter.

Implements the formatter for all string types.
[format.formatter.spec]/2.2
For each charT, the string type specializations
```
template<> struct formatter<c

[libc++][format] Adds string formatter.

Implements the formatter for all string types.
[format.formatter.spec]/2.2
For each charT, the string type specializations
```
template<> struct formatter<charT*, charT>;
template<> struct formatter<const charT*, charT>;
template<size_t N> struct formatter<const charT[N], charT>;
template<class traits, class Allocator>
struct formatter<basic_string<charT, traits, Allocator>, charT>;
template<class traits>
struct formatter<basic_string_view<charT, traits>, charT>;
```
This removes the stub implemented in D96664.

Implements parts of:
- P0645 Text Formatting
- P1868 width: clarifying units of width and precision in std::format

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103425

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