Request LLVM to implement _CountLeadingZeros and _CountLeadingZeros64 when targeting ARM/ARM64

[cbezault]

Currently LLVM does not define the MSVC builtins _CountLeadingZeros and _CountLeadingZeros64 when targeting ARM/ARM64. This forces us to maintain a parallel code path in <bit> for LLVM targeting ARM.

We should make a bug/feature request asking the LLVM devs to implement these builtins when targeting ARM/ARM64.

[AraHaan]

Why not request it now?

[AlexGuteniev]

Can you use __builtin_clz / __builtin_clzll?

https://godbolt.org/z/G4cdsz

[AlexGuteniev]

Ah, you use them already, but want to get rid of this separation

[AraHaan]

Why not alias those as _ClountLeadingZeros and _CountLeadingZeros64 on clang when using ARM / ARM64 instead?

[frederick-vs-ja]

LLVM-66554 added these intrinsics to Clang 18.