[llvm] [ADT] Teach StringRef to take basic features from std::string_view (NFC) (PR #113797)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Seems that there is no conclusion on the approach. https://github.com/llvm/llvm-project/pull/113775 is reverted
https://discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785

https://github.com/llvm/llvm-project/pull/113797