[clang] Revert "[Headers][X86] Allow SSE/AVX/AVX512 unpck intrinsics to be used in constexpr" (PR #153491)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

If you can get your fix in immediately that's fine. But we shouldn't leave bots red for so long, and normally I'd have expected someone to have already pushed a revert by now.

https://github.com/llvm/llvm-project/pull/153491