[libc++][NFC] Add missing license headers

Also standardize the license comment in several files where it was
different from what we normally do.

[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduc

[libc++] Mark slow tests as unsupported on GCC

Some tests in our test suite are unbelievably slow on GCC due to the
use of the always_inline attribute. See [1] for more details.

This patch introduces the GCC-ALWAYS_INLINE-FIXME lit feature to
disable tests that are plagued by that issue. At the same time, it
moves several existing tests from ad-hoc `UNSUPPORTED: gcc-12` markup
to the new GCC-ALWAYS_INLINE-FIXME feature, and marks the slowest tests
reported by the CI as `UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME`.

[1]: https://discourse.llvm.org/t/rfc-stop-supporting-extern-instantiations-with-gcc/71277/1

Differential Revision: https://reviews.llvm.org/D152736

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[libc++][format] Removes the experimental status.

The code has been quite ready for a while now and there are no more ABI
breaking papers. So this is a good time to mark the feature as stable.

Revi

[libc++][format] Removes the experimental status.

The code has been quite ready for a while now and there are no more ABI
breaking papers. So this is a good time to mark the feature as stable.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D150802

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[libc++] Use named Lit features to flag back-deployment XFAILs

Instead of writing something like `XFAIL: use_system_cxx_lib && target=...`
to XFAIL back-deployment tests, introduce named Lit feature

[libc++] Use named Lit features to flag back-deployment XFAILs

Instead of writing something like `XFAIL: use_system_cxx_lib && target=...`
to XFAIL back-deployment tests, introduce named Lit features like
`availability-shared_mutex-missing` to represent those. This makes the
XFAIL annotations leaner, and solves the problem of XFAIL comments
potentially getting out of sync. This would also make it easier for
another vendor to add their own annotations to the test suite by simply
changing how the feature is defined for their OS releases, instead
of having to modify hundreds of tests to add repetitive annotations.

This doesn't touch *all* annotations -- only annotations that were widely
duplicated are given named features (e.g. when filesystem or shared_mutex
were introduced). I still think it probably doesn't make sense to have a
named feature for every single fix we make to the dylib.

This is in essence a revert of 2659663, but since then the test suite
has changed significantly. Back when I did 2659663, the configuration
files we have for the test suite right now were being bootstrapped and
it wasn't clear how to provide these features for back-deployment in
that context. Since then, we have a streamlined way of defining these
features in `features.py` and that doesn't impact the ability for a
configuration file to stay minimal.

The original motivation for this change was that I am about to propose
a change that would touch essentially all XFAIL annotations for back-deployment
in the test suite, and this greatly reduces the number of lines changed
by that upcoming change, in addition to making the test suite generally
better.

Differential Revision: https://reviews.llvm.org/D146359

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[libc++] Remove availability markup for std::format

std::format is currently experimental, so there is technically no
deployment target requirement for it (since the only symbols required
for it are

[libc++] Remove availability markup for std::format

std::format is currently experimental, so there is technically no
deployment target requirement for it (since the only symbols required
for it are in `libc++experimental.a`).

However, some parts of std::format depend indirectly on the floating
point std::to_chars implementation, which does have deployment target
requirements.

This patch removes all the availability format for std::format and
updates the XFAILs in the tests to properly explain why they fail
on old deployment targets, when they do. It also changes a couple
of tests to avoid depending on floating-point std::to_chars when
it isn't fundamental to the test.

Finally, some tests are marked as XFAIL but I added a comment saying

TODO FMT This test should not require std::to_chars(floating-point)

These tests do not fundamentally depend on floating-point std::to_chars,
however they end up failing because calling std::format even without a
floating-point argument to format will end up requiring floating-point
std::to_chars. I believe this is an implementation artifact that could
be avoided in all cases where we know the format string at compile-time.
In the tests, I added the TODO comment only to the places where we could
do better and actually avoid relying on floating-point std::to_chars
because we know the format string at compile-time.

Differential Revision: https://reviews.llvm.org/D134598

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Reapply "[libc++][ranges]Refactor `copy{,_backward}` and `move{,_backward}`"

This reverts commit a6e1080b87db8fbe0e1afadd96af5a3c0bd5e279.

Fix the conditions when the `memmove` optimization can be

Reapply "[libc++][ranges]Refactor `copy{,_backward}` and `move{,_backward}`"

This reverts commit a6e1080b87db8fbe0e1afadd96af5a3c0bd5e279.

Fix the conditions when the `memmove` optimization can be applied and refactor them out into a reusable type trait, fix and significantly expand the tests.

Differential Revision: https://reviews.llvm.org/D139235

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[libc++][format] Implements formattable concept.

This concept is introduced in P2286, but was implemented in libc++
before. This implementation was used in the library internally. This
implementatio

[libc++][format] Implements formattable concept.

This concept is introduced in P2286, but was implemented in libc++
before. This implementation was used in the library internally. This
implementation lacked the resolution of LWG3636. The original formatter
had a non-const member function that wasn't trivial to make a const
member. The recent parser improvements made this member a const member
in preparation of LWG3636.

Note LWG3636 isn't voted in. Its status is Ready. P2286's concept has
been written as-if LWG3636 is accepted and refers to that LWG issue.

Updates some tests make format a const member function and removes a
tests that's mainly a duplicate of the formattable concept test.

Implements
- LWG3636 formatter<T>::format should be const-qualified

Implements parts of
- P2286R8 Formatting Ranges

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D134110

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[libc++][format] Use forwarding references.

This implements a not accepted LWG issue. Not doing so would require
integral types to use the handle class instead of being directly stored
in the basic_

[libc++][format] Use forwarding references.

This implements a not accepted LWG issue. Not doing so would require
integral types to use the handle class instead of being directly stored
in the basic_format_arg.

The previous code used `std::forward` in places where it wasn't required
by the Standard. These are now removed.

Implements:
- P2418R2 Add support for std::generator-like types to std::format
- LWG 3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D127570

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