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

[via llvm-commits]

trcrsired 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

wincall should not do anything to i686 but only x86_64

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