[clang] [llvm] [HLSL] Implement elementwise popcount (PR #108121)

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon approved this pull request.

LGTM after @farzonl minors have been addressed - once this is committed I'm intending to use `__builtin_elementwise_popcount` to simplify a number of C++ target intrinsics that do the same thing.

https://github.com/llvm/llvm-project/pull/108121