[libclc] [NFC][libclc] Refine clz to use __builtin_clzg (PR #135301)

[Wenju He via cfe-commits]

wenju-he wrote:

> Note I'm in the process of introducing elementwise clz/ctz builtins which would accomplish this and achieve vector intrinsics. See #131995. It might be worth waiting for that change instead?

thanks @frasercrmck 
LGTM. Please refactor clz after #131995
close this PR.

https://github.com/llvm/llvm-project/pull/135301