[clang] [DiagnosticInfo] Allow std::string_view in DiagnosticBuilder operator<<. (PR #190374)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 commented:

Can we get a unit test somewhere that exercises this overload with a comment that it's used downstream so that this doesn't get deleted later by virtue of being unused in tree?

https://github.com/llvm/llvm-project/pull/190374