This applies clang-format to the std::string unit tests in preparation
for landing https://reviews.llvm.org/D140550.

Differential Revision: https://reviews.llvm.org/D140612

This has been done using the following command

  find libcxx/test -type f -exec perl -pi -e 's|^([^/]+?)((?<!::)size_t)|\1std::\2|' \{} \;

And manually removed some false positives in std/depr/depr.c.headers.

The `std` module doesn't export `::size_t`, this is a preparation for that module.

Reviewed By: ldionne, #libc, EricWF, philnik

Differential Revision: https://reviews.llvm.org/D146088

The `basic_string_view` constructor accepting a contiguous range rejects
converting between `basic_string_view` even when only the trait types vary.
This prevents conversions for converting from `basic_string_view<C, T1>` and
`basic_string<C, T1, A>` to `basic_string_view<C, T2>`.  Recently, this
constructor was made `explicit`, so there's no reason to really forbid this
conversion anymore.

Relax the restriction that the trait types need to match in this constructor.

Differential Revision: https://reviews.llvm.org/D143972

When we ship LLVM 16, <ranges> won't be considered experimental anymore.
We might as well do this sooner rather than later.

Differential Revision: https://reviews.llvm.org/D132151

Reviewed By: #libc, philnik, Mordante

Spies: Mordante, jloser, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D130785

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

Remove `s.base()`; every test that wants to get the base of a "test sentinel"
should use the ADL `base(s)` from now on.

Differential Revision: https://reviews.llvm.org/D115766

Implement P1989R2 which adds a range constructor for `string_view`.

Adjust `operator/=` in `path` to avoid atomic constraints caching issue
getting provoked from this PR.

Add defaulted template argument to `string_view`'s "sufficient
overloads" to avoid mangling issues in `clang-cl` builds. It is a
MSVC mangling bug that this works around.

Differential Revision: https://reviews.llvm.org/D113161