[llvm] [ADT] Teach StringRef to derive from std::string_view (PR #113752)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This patch was posted before https://discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785 . I thought if StringRef could derive from std::string_view, it would be really nice to simplify interop with std::string_view, whether or not we decide to keep StringRef (perhaps we would only use StringRef as temporary objects for its specific APIs, instead of storing StringRef in data structures). However, there were many issues due to GCC overload resolution.

https://github.com/llvm/llvm-project/pull/113752