[clang] [Headers][X86] Allow FMA intrinsics to be used in constexpr (PR #156385)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon approved this pull request.

LGTM - the mask3 intrinsics seem to be mixed up randomly and could do with sorting out at some point to be consistently with their siblings, but that's not relevant for this patch.

https://github.com/llvm/llvm-project/pull/156385