Partially revert libc++ commit aa7f377c965c, restoring stdint.h

This partially reverts upstream libc++ commit aa7f377c965c:

[libc++] Remove libc++'s own stdint.h and locale.h (#107436)

These h

Partially revert libc++ commit aa7f377c965c, restoring stdint.h

This partially reverts upstream libc++ commit aa7f377c965c:

[libc++] Remove libc++'s own stdint.h and locale.h (#107436)

These headers are not doing anything beyond the system or compiler
provided equivalent headers, so there's no real reason to keep them
around. Reducing the number of C headers we provide in libc++ simplifies
our header layering and reduces the potential for confusion when headers
are layered incorrectly.

The problem is that libc++'s own `stdint.h` _does_ do something: it
defines `__STDC_LIMIT_MACROS` and `__STDC_CONSTANT_MACROS` before
including the system `stdint.h`, causing the latter to expose macros
like `SIZE_MAX`, `UINT64_C`, and others.

If the `__STDC_LIMIT_MACROS` and `__STDC_CONSTANT_MACROS` macros are not
exposed, C++ programs compiled for standards before C++11 can fail with
errors due to those macros not being available.

PR: 292067
MFC after: 1 month

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Merge llvm-project release/21.x llvmorg-21.1.7-0-gcd708029e0b2

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/21.x llvmorg-21.1.7-0-gcd708029e

Merge llvm-project release/21.x llvmorg-21.1.7-0-gcd708029e0b2

This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
openmp to llvm-project release/21.x llvmorg-21.1.7-0-gcd708029e0b2,
a.k.a. 21.1.7 release.

PR: 292067
MFC after: 1 month

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Fixups after llvm-project main llvmorg-21-init-19288-gface93e724f4 merge

Fix various configuration fails, update generated headers, Makefiles, etc.

PR: 292067
MFC after: 1 month