[clang] [Clang] Introduce scoped variants of GNU atomic functions (PR #72280)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Missing change to clang/docs/LanguageExtensions.rst describing the new builtins.
>
Will do.

> Are there any other projects that we might want to coordinate with here? gcc, maybe?

Unknown, I've never collaborated with anyone outside of LLVM. I know they have handling of GPU programming for NVPTX and AMDGPU targets, but I don't know what level of support they have for this. I think it's sufficient to keep it as a `clang` extension for now. I'm hoping this is relatively simple given it's just an extra argument on the GNU versions. 

https://github.com/llvm/llvm-project/pull/72280