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

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

> Can you move the ADT changes to a separate PR and add unit tests?

@kuhar Will do. I'll wait for some reviews on the usage of that ADT change to be sure that it's actually going to be required for this and then create that separate PR.

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