[clang] [Headers][X86] Allow AVX512 masked shuffles to be used in constexpr (PR #162301)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon requested changes to this pull request.

still quite a few missing tests - and need to avoid using "0xFF" allbits" style masks

https://github.com/llvm/llvm-project/pull/162301