[clang] [clang] Enable constexpr handling for __builtin_elementwise_fma (PR #152919)

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

@RKSimon Done. Also rebased against main.

I ignored the clang-format warning because the other version looked better IMO, but I've updated it now.

https://github.com/llvm/llvm-project/pull/152919