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

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

Indeed. I considered rearranging them, and the tests too, but chose to leave it for another time to not make this patch even longer.

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