[clang] [llvm] [HLSL] implement elementwise firstbithigh hlsl builtin (PR #111082)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

This looks really good. I hade a few minor things and questions about either pulling things out of this pr and putting them into their own changes or future clean up to consolidate things. One more iteration and I'll be happy to approve.

https://github.com/llvm/llvm-project/pull/111082