libc++: Fix deprecation warnings with latest clang

[arichardson]

Summary

Latest clang has become more strict in diagnosing deprecated decls, so
pull in LLVM commit 9feac2cbd0d80927ce9a8b4c3e810d2b81802d55.

Original commit message:
[libc++] Improve deprecated diagnostic guards.

Recent Clang-21 builds improved the deprecated diagnotics. This
uncovered missing guards in libc++ internally.

Note: This patch should be a separate commit and not merged.
For testing purposes they are combined.

[emaste]

Note: This patch should be a separate commit and not merged.
For testing purposes they are combined.

This seems confusing in the commit message

[arichardson]

In D52531#1199810, @emaste wrote:

Note: This patch should be a separate commit and not merged.
For testing purposes they are combined.

This seems confusing in the commit message

will drop that part of the commit message, I guess something went wrong when this was committed upstream.