[libc++] Refactor the configuration macros to being always defined (#112094)

This is a follow-up to #89178. This updates the `<__config_site>`
macros.

[libc++] Mark P2845R8 `__cpp_lib_format_path` and P2587R3 `__cpp_lib_to_string` as C++26 (#93255)

[P2845R8](https://wg21.link/P2845R8) "Formatting of
`std::filesystem::path`" and [P2587R3](https://

[libc++] Mark P2845R8 `__cpp_lib_format_path` and P2587R3 `__cpp_lib_to_string` as C++26 (#93255)

[P2845R8](https://wg21.link/P2845R8) "Formatting of
`std::filesystem::path`" and [P2587R3](https://wg21.link/P2587R3)
"`to_string` or not `to_string`" are C++26 features, so they should be
marked accordingly in `generate_feature_test_macro_components.py`.

I verified that without my changes, running the script produced no
edits. Then with my changes, I ran the script to regenerate all files,
with no other manual edits.

Found while running libc++'s tests with MSVC's STL, which noticed this
because it's currently a C++23-only implementation.

Note that @H-G-Hristov has a draft implementation of P2587R3: #78100

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[libc++] Update status page after the Tokyo meeting (#87395)

[libc++] Add missing conditionals for feature-test macros (#80168)

We noticed that some feature-test macros were not conditional on
configuration flags like _LIBCPP_HAS_NO_FILESYSTEM. As a result,

[libc++] Add missing conditionals for feature-test macros (#80168)

We noticed that some feature-test macros were not conditional on
configuration flags like _LIBCPP_HAS_NO_FILESYSTEM. As a result, code
attempting to use FTMs would not work as intended.

This patch adds conditionals for a few feature-test macros, but more
issues may exist.

rdar://122020466

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[libc++][NFC] Refactor _LIBCPP_AVAILABILITY_HAS_* macros to always be defined (#71002)

This makes the conditionals quite a bit simpler to understand, since it
avoids double negatives and makes sure

[libc++][NFC] Refactor _LIBCPP_AVAILABILITY_HAS_* macros to always be defined (#71002)

This makes the conditionals quite a bit simpler to understand, since it
avoids double negatives and makes sure we have <__availability>
included. For vendors which use availability macros, it also enforces
that they check when specific features are introduced and define the
macro for their platform appropriately.

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[libc++] Rename availability macros for <filesystem>

This makes it clearer that the availability macro only pertains to
<filesystem>, and not to whether the platform has support for a file
system.

[libc++] Rename availability macros for <filesystem>

This makes it clearer that the availability macro only pertains to
<filesystem>, and not to whether the platform has support for a file
system.

Differential Revision: https://reviews.llvm.org/D152172

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[libc++] Adds C++26 support.

Clang has been updated to support C++26, this adds the same support for
libc++. At the moment C++23 and C++26 are identical. During the next
plenary in June the first C+

[libc++] Adds C++26 support.

Clang has been updated to support C++26, this adds the same support for
libc++. At the moment C++23 and C++26 are identical. During the next
plenary in June the first C++26 papers will be voted on.

Note like Clang this patch uses C++26 is the internal part and C++2c in
the user visible part.

Depends on D150795

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D151026

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[libc++] Updates C++2b to C++23.

During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.

This updates the reference to c++2b to c++23 and updat

[libc++] Updates C++2b to C++23.

During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.

This updates the reference to c++2b to c++23 and updates the __cplusplus
macro.

Note since we use clang-tidy 16 a small work-around is needed. Clang
knows -std=c++23 but clang-tidy not so for now force the lit compiler
flag to use -std=c++2b instead of -std=c++23.

Reviewed By: #libc, philnik, jloser, ldionne

Differential Revision: https://reviews.llvm.org/D150795

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[libc++] Systematically define a macro to check whether a feature is unavailable

Previously, we inconsistently defined whether a feature was unavailable
on a given deployment target. We would have a

[libc++] Systematically define a macro to check whether a feature is unavailable

Previously, we inconsistently defined whether a feature was unavailable
on a given deployment target. We would have availability attributes for
all features, but only some features had the equivalent of a _HAS_NO_FOO
macro in the form of the DISABLE_FTM___foo macros. Instead, systematically
define a _HAS_NO_FOO macro, which makes it easier to understand how to
add availability markup for a new platform.

Differential Revision: https://reviews.llvm.org/D147226

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[libc++] Improves feature-test macro diagnostics.

This was mentioned in review D131326.

Reviewed By: var-const, #libc, philnik

Differential Revision: https://reviews.llvm.org/D132293

[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

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

[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D129195

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[runtimes] Rename various libcpp-has-no-XYZ Lit features to just no-XYZ

Since those features are general properties of the environment, it makes
sense to use them from libc++abi too, and so the name

[runtimes] Rename various libcpp-has-no-XYZ Lit features to just no-XYZ

Since those features are general properties of the environment, it makes
sense to use them from libc++abi too, and so the name libcpp-has-no-xxx
doesn't make sense.

Differential Revision: https://reviews.llvm.org/D126482

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[libc++] Make .version.pass.cpp tests be compile-only tests

We don't really need to run them.