[clang] [z/OS][Clang] Add compare and swap builtin functions (PR #202362)

[Sean Perry via cfe-commits]

https://github.com/perry-ca commented:

looking good.  

Can you document what these builtins expand into.  I think the right place is clang/docs/LanguageExtensions.rst.

These should interact with the -fno-builtin and -fn-builtin-<value> options.  Can you add test cases to cover this.

https://github.com/llvm/llvm-project/pull/202362