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

[Jon Chesterfield via cfe-commits]

https://github.com/JonChesterfield approved this pull request.

This is functionally correct and useful as is - if gcc decide to do something divergent we can change it later, it's basically an internal interface anyway. Let's have it now and change the names if we come up with better ideas later.

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