[clang] [Headers][X86] Use `__builtin_elementwise_ctlz` instead of avx512cd intrinsics. (PR #155089)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

> > Convert _mm_lzcnt_epi32 etc. (inc mask/maskz variants) to use __builtin_elementwise_ctlz
> 
> It's not obvious to me how the mask/maskz variants (e.g. `_mm_maskz_lzcnt_epi32`) can be refactored to use `__builtin_elementwise_ctlz`.
> 
> Is it enough to replace this call:
> 
> https://github.com/llvm/llvm-project/blob/35e733689b9b47469cbb51a90d405738146e447c/clang/lib/Headers/avx512cdintrin.h#L84
> 
> by a call to `__builtin_elementwise_ctlz`?

No, calling to intrinsic is good. We just need to change `__DEFAULT_FN_ATTRS` to `__DEFAULT_FN_ATTRS_CONSTEXPR` and add tests for them.

https://github.com/llvm/llvm-project/pull/155089