[libcxx-commits] [libcxx] [libc++] refactor atomic_waitable_traits into its own header and remove pre 20 support (PR #173157)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne approved this pull request.

LGTM, I edited the PR description to explain the rationale for this change.

https://github.com/llvm/llvm-project/pull/173157