[clang] [clang] Add per-global code model attribute (PR #72078)

[WÁNG Xuěruì via cfe-commits]

https://github.com/xen0n commented:

Overall this seems fine, thanks!

But I'm not sure if reporting a more generic "the model attribute is not supported on this target" for non-LoongArch would be better: it doesn't give the false impression that the target doesn't support the specified code model.

https://github.com/llvm/llvm-project/pull/72078