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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Please wait for feedback on [discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785/2](https://discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785/2)

While the broader discussion of StringRef migration or string_view integration is debatable, this specific member function migration seems reasonable. The substr function is well-established and widely understood, making this patch a worthwhile improvement. There is certainly some churn cost associated with such patches, but I believe it's acceptable in this case.

https://github.com/llvm/llvm-project/pull/113793