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

[Farzon Lotfi via cfe-commits]

farzonl wrote:

Also we probably still want a `clang/test/SemaHLSL/BuiltIns/countbits-errors.hlsl` to check the errors for float representations.

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