[libc++][nfc] Add TEST_HAS_NO_THREADS.

This avoids using an libc++ internal macro in our tests.

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D119563

[libc++] Enable <atomic> when threads are disabled

std::atomic is, for the most part, just a thin veneer on top of compiler
builtins. Hence, it should be available even when threads are not availabl

[libc++] Enable <atomic> when threads are disabled

std::atomic is, for the most part, just a thin veneer on top of compiler
builtins. Hence, it should be available even when threads are not available
on the system, and in fact there has been requests for such support.

This patch:
- Moves __libcpp_thread_poll_with_backoff to its own header so it can
be used in <atomic> when threads are disabled.
- Adds a dummy backoff policy for atomic polling that doesn't know about
threads.
- Adjusts the <atomic> feature-test macros so they are provided even when
threads are disabled.
- Runs the <atomic> tests when threads are disabled.

rdar://77873569

Differential Revision: https://reviews.llvm.org/D114109

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[libc++] Add an option to disable wide character support in libc++

Some embedded platforms do not wish to support the C library functionality
for handling wchar_t because they have no use for it. It

[libc++] Add an option to disable wide character support in libc++

Some embedded platforms do not wish to support the C library functionality
for handling wchar_t because they have no use for it. It makes sense for
libc++ to work properly on those platforms, so this commit adds a carve-out
of functionality for wchar_t.

Unfortunately, unlike some other carve-outs (e.g. random device), this
patch touches several parts of the library. However, despite the wide
impact of this patch, I still think it is important to support this
configuration since it makes it much simpler to port libc++ to some
embedded platforms.

Differential Revision: https://reviews.llvm.org/D111265

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[libc++] Build and test with -Wundef warning. NFC.

This will avoid typos like `_LIBCPP_STD_VERS` (<future>) or using `#if TEST_STD_VER > 17` without including "test_macros.h".

Reviewed By: ldionne,

[libc++] Build and test with -Wundef warning. NFC.

This will avoid typos like `_LIBCPP_STD_VERS` (<future>) or using `#if TEST_STD_VER > 17` without including "test_macros.h".

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D99515

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[libc++] [P0482] [C++20] Implement missing bits for atomic

Added: ATOMIC_CHAR8_T_LOCK_FREE, atomic<char8_t>, atomic_char8_t.
http://wg21.link/P0482

Reviewed By: ldionne, #libc

Differential Revisio

[libc++] [P0482] [C++20] Implement missing bits for atomic

Added: ATOMIC_CHAR8_T_LOCK_FREE, atomic<char8_t>, atomic_char8_t.
http://wg21.link/P0482

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D91706

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[libc++][test] atomic<T> requires trivially copyable T

The author of these tests apparently forgot that `atomic_{,un}signed_lock_free` are already specializations of `atomic`.

[libc++] Add comment in atomic test to explain why part of it is disabled on Apple

Update atomic feature macros, synopsis, signatures to match C++20. Improve test coverage for non-lock-free atomics.

[libc++] Workaround unused variable warning in test

This only showed up in C++11/C++14 where the static_assert below was
ifdef'd out, and the variable was indeed unused.

[libc++] Fix CI and Linux failures after landing D68480

- Avoid using C++11-and-later features in <atomic>:
Historically, we've supported <atomic> in C++03, so we can't use C++11
features in tha

[libc++] Fix CI and Linux failures after landing D68480

- Avoid using C++11-and-later features in <atomic>:
Historically, we've supported <atomic> in C++03, so we can't use C++11
features in that header. This is something we really need to change,
since our implementation of <atomic> is starting to accumulate technical
debt because of that.
- Mark a test as unsupported on single threaded systems
- Add missing symbols to the Linux ABI list

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[libc++] Implementation of C++20's P1135R6 for libcxx

Differential Revision: https://reviews.llvm.org/D68480