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

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon requested changes to this pull request.

The mask/maskz tests shouldn't be a problem - __builtin_ia32_select* intrinsics are already constexpr - you just need to add tests similar to what we've done in files lie avx512vpopcntdq-builtins.c

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