[libc++] Update atomic synopsis and tests.

[libc++] Update atomic synopsis and tests.

While looking at LWG-2988 and P0558 it seems the issues were already
implemented, but the synopsis wasn't updated. Some of the tests didn't
validate the `noexcept` status. A few tests were missing completely:
- `atomic_wait_explicit`
- `atomic_notify_one`
- `atomic_notify_all`

Mark P0558 as complete, didn't investigate which version of libc++ first
includes this. It seems the paper has been retroactively applied. I
couldn't find whether this is correct, but looking at cppreference it
seems intended.

Completes
- LWG-2988 Clause 32 cleanup missed one typename
- P0558 Resolving atomic<T> named base class inconsistencies

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D103765