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

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

@RKSimon Not much is left after #155405. I need to clean up the type checking assertions in the new interpreter, probably just remove them. I'll finish it soon.

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