[HLSL] Change WaveActiveCountBits to wrapper of __builtin_hlsl_wave_active_count_bits

Change WaveActiveCountBits from builtin into wrapper of __builtin_hlsl_wave_active_count_bits.
For comment at
https://reviews.llvm.org/D126857#inline-1235949

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D128855