[clang] [Headers][X86] Allow SSE2/AVX2/AVX512F/AVX512BW/AVX512DQ integer arithmetic intrinsics to be used in constexpr (PR #157582)

[Simon Pilgrim via cfe-commits]

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

The next (tedious) step is to add test coverage - see #156369 for examples

https://github.com/llvm/llvm-project/pull/157582