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

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

Note I'm in the process of introducing elementwise clz/ctz builtins which would accomplish this and achieve vector intrinsics. See https://github.com/llvm/llvm-project/pull/131995. It might be worth waiting for that change instead?

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