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

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Andarwinux wrote:

APX (EVEX) also implies AVX512, which means all instructions are expected VEX/EVEX encoded. Can we eliminate the use of vzeroupper under WinCall based on this?

Also, based assumptions of AVX512, can we make better use of ZMM and ZMM16-31 in WinCall Calling Convention?

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