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

[Maryam Moghadas via cfe-commits]

maryammo wrote:

> LanguageExtensions.rst.

As far as I see, the `-fno-builtin` option only affects standard library functions (memcpy, strlen, etc.). Compiler/target-specific builtins don't interact with `-fno-builtin`.

In Builtins.cpp, the -fno-builtin-foo unregistration code checks `isPredefinedLibFunction(ID)` (which determines whether this builtin is a predefined libc/libm function such as mallo; has the 'f' attribute) before clearing the builtin ID. __cs* are not predefined lib functions.

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