[clang] [llvm] [X86] Add WinCall Calling Convention and x86_64apx-windows(-gnu/-msvc) triplets (PR #215585)

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void2012 wrote:

I think on 32-bit x86 there was a `mregparm=3`, can something like this be introduced on x64 or does this draft cc defeat the purpose of such parameters

https://github.com/llvm/llvm-project/pull/215585