[llvm] [clang] [SystemZ] Properly support 16 byte atomic int/fp types and ops. (PR #73134)

[Ulrich Weigand via cfe-commits]

https://github.com/uweigand commented:

Not sure about the is_lock_free results, see inline comments.

What does recent GCC return for those?

https://github.com/llvm/llvm-project/pull/73134