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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> The underlying implementation is a string literal in the LLVM syncscope argument, but the problem is that this isn't standardized at all and varies between backends potentially

We don't have to use the same set of strings as syncscope if that doesn't make sense.

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