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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

We're already assigning names to the different scopes; we're just doing it in the __MEMORY_SCOPE_* preprocessor macros.  Replacing `_MEMORY_SCOPE_SYSTEM` in the code with `"system"` doesn't really change the nature of how we assign names to scopes.

If these are supposed to match the OpenCL enum values, though, I guess that's an argument for using numbers.

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