[libc++] Make the Debug mode a configuration-time only option

The debug mode has been broken pretty much ever since it was shipped
because it was possible to enable the debug mode in user code witho

[libc++] Make the Debug mode a configuration-time only option

The debug mode has been broken pretty much ever since it was shipped
because it was possible to enable the debug mode in user code without
actually enabling it in the dylib, leading to ODR violations that
caused various kinds of failures.

This commit makes the debug mode a knob that is configured when
building the library and which can't be changed afterwards. This is
less flexible for users, however it will actually work as intended
and it will allow us, in the future, to add various kinds of checks
that do not assume the same ABI as the normal library. Furthermore,
this will make the debug mode more robust, which means that vendors
might be more tempted to support it properly, which hasn't been the
case with the current debug mode.

This patch shouldn't break any user code, except folks who are building
against a library that doesn't have the debug mode enabled and who try
to enable the debug mode in their code. Such users will get a compile-time
error explaining that this configuration isn't supported anymore.

In the future, we should further increase the granularity of the debug
mode checks so that we can cherry-pick which checks to enable, like we
do for unspecified behavior randomization.

Differential Revision: https://reviews.llvm.org/D122941

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[NFC][libc++][format] Improves naming.

Based on review comments in D110499.

Depends on D110499

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D125610

[libc++][format][5/6] Improve format_to_n.

Use a specialized buffer wrapper to limit the number of insertions in the
buffer. After the limit has been reached the buffer only needs to count
the numbe

[libc++][format][5/6] Improve format_to_n.

Use a specialized buffer wrapper to limit the number of insertions in the
buffer. After the limit has been reached the buffer only needs to count
the number of insertions to return the buffer size required to store the
entire output.

Depends on D110498

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D110499

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[libc++][format][4/6] Improve formatted_size.

Use a specialized "buffer" to count the number of insertions instead of
using a `string` as storage type.

Depends on D110497.

Reviewed By: ldionne, #l

[libc++][format][4/6] Improve formatted_size.

Use a specialized "buffer" to count the number of insertions instead of
using a `string` as storage type.

Depends on D110497.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D110498

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[libc++][format][3/6] Adds a __container_buffer.

Instead of writing every character directly into the container by using
a `back_insert_iterator` the data is buffered in an `array`. This buffer
is t

[libc++][format][3/6] Adds a __container_buffer.

Instead of writing every character directly into the container by using
a `back_insert_iterator` the data is buffered in an `array`. This buffer
is then inserted to the container by calling its `insert` member function.

Since there's no guarantee every container's `insert` behaves properly
containers need to opt-in to this behaviour. The appropriate standard
containers opt-in to this behaviour.

This change improves the performance of the format functions that use a
`back_insert_iterator`.

Depends on D110495

Reviewed By: ldionne, vitaut, #libc

Differential Revision: https://reviews.llvm.org/D110497

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[libc++][format][2/6] Adds a __output_iterator.

Instead of using a temporary `string` in `__vformat_to_wrapped` use a new
generic iterator. This aids to reduce the number of template instantions
and

[libc++][format][2/6] Adds a __output_iterator.

Instead of using a temporary `string` in `__vformat_to_wrapped` use a new
generic iterator. This aids to reduce the number of template instantions
and avoids using a `string` to buffer the entire formatted output.

This changes the type of `format_context` and `wformat_context`, this can
still be done since the code isn't ABI stable yet.

Several approaches have been evaluated:
- Using a __output_buffer base class with:
- a put function to store the buffer in its internal buffer
- a virtual flush function to copy the internal buffer to the output
- Using a `function` to forward the output operation to the output buffer,
much like the next method.
- Using a type erased function point to store the data in the buffer.
The last version resulted in the best performance. For some cases there's
still a loss of speed over the original method. This loss many becomes
apparent when large strings are copied to a pointer like iterator, before
the compiler optimized this using `memcpy`.

Reviewed By: ldionne, vitaut, #libc

Differential Revision: https://reviews.llvm.org/D110495

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