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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

My primary concerns here are:
- It being likely these builtins will be superseded by something else once someone else tries to standardize this.  Maybe this isn't a big deal... but maybe we want to choose names that are less likely to overlap with stuff anyone else is doing.
- We should try to avoid new preprocessor macros if possible; these are going to get unconditionally injected into every translation unit anyone ever builds with clang in any language mode, so we want to try to avoid extending the number of macros we define where possible.

That said, I don't want to block progress here; this is clearly important functionality to have.

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