[clang] [clang][clangIR]: Implement builtin x86 arch cpu detection. (PR #212143)

[Erich Keane via cfe-commits]

erichkeane wrote:

IF you were interested in something that shouldn't be too difficult/awful, perhaps check out the bug I just reported? https://github.com/llvm/llvm-project/issues/213024 

I believe it is a problem in LowerToLLVM where we aren't properly propagating no_proto, so it is perhaps reasonably easy (though maybe a good amount of tests to update?).  I won't be working on it in the near future, but if you wanted to claim that, I'll help how I can.

https://github.com/llvm/llvm-project/pull/212143