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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

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

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

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