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

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

> What about integers and integer vectors?

I believe `__builtin_elementwise_fma` only accepts direct floating point numbers and vectors of floating point numbers as arguments, so we don't need to handle integers.

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