[libc++][P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

This is a re-application of 5d1c1a24, which was reverted in 987c7f40


because it broke the LLDB build.

Co-authored-by: Marek Kurdej <marek.kurdej@gmail.com>

Differential Revision: https://reviews.llvm.org/D97044