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

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heiher wrote:

> 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.

In my opinion I think this is better. I put it in a separate commit so we can easily revert it if someone thinks otherwise.

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