[clang] [llvm] [HLSL] Implement elementwise firstbitlow builtin (PR #116858)

[Steven Perron via cfe-commits]

s-perron wrote:

The spir-v test failures are unrelated to your change. It is caused by a problem in spirv-val, and it has been fixed.Don't let that stop you from merging.

https://github.com/llvm/llvm-project/pull/116858