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

[Jackson Stogel via cfe-commits]

jtstogel wrote:

> 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?

Good idea, done. Also, moved around some helpers in `clang/unittests/Basic/DiagnosticTest.cpp` and updated `WithMessage` to accept a matcher.

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