[clang] [llvm] [Clang] Enable constexpr handling for builtin elementwise fshl/fshr (PR #153572)

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

Sounds like the implementation of `fshl/r` looks OK. Let me create another PR for the ADT changes like kuhar suggested, I'll add the fshl/r functions there.

https://github.com/llvm/llvm-project/pull/153572