[clang] [llvm] [Support] Use CTLog2 from PointerLikeTypeTraits.h (NFC) (PR #157790)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> ConstantLog2 name is better though

@arsenm Yeah, `CTLog2` is a bit cryptic.  Let me follow up by renaming `CTLog2` to `ConstantLog2` after this patch.  Thanks for reviews as always.

https://github.com/llvm/llvm-project/pull/157790