[llvm] [llvm] Prefer StringRef::substr(0, N) to slice(0, N) (NFC) (PR #113793)

[David Blaikie via llvm-commits]

dwblaikie wrote:

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/113793