[clang] [clang] Prefer StringRef::substr(0, N) to slice(0, N) (NFC) (PR #113784)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Should we agree on #113793, and then take the same decision taken here?

I think we can go ahead with this. dblaikie has a comment there at #113793 

> Yeah, converging the APIs where there's not huge differences (in this case we support both, one is a standard name/behavior) seems like a win for usability even if we never coalesce StringRef with string_view.

https://github.com/llvm/llvm-project/pull/113784