[libcxx-commits] [libcxx] [libc++] Deprecate the C++20 synchronization library before C++20 (PR #86410)

[Louis Dionne via libcxx-commits]

ldionne wrote:

This is the deprecation step before #82008.

Note that this PR is incomplete, I'll have another pass at it.

https://github.com/llvm/llvm-project/pull/86410