[libc++] Refactor tests for trivially copyable atomics

- Replace irrelevant synopsis by a comment
- Use a .verify.cpp test instead of .compile.fail.cpp
- Remove unnecessary includes in one of the tests (was a copy-paste error)

Differential Revision: https://reviews.llvm.org/D114094

GitOrigin-RevId: 3e957e5d66aaab08c33ccea2768f74993f6f554f
4 files changed