[clang] [NFC][Clang] Refactor ClangDiagnosticEmitter to use more StringRef (PR #115212)

[Kazu Hirata via cfe-commits]

https://github.com/kazutakahirata commented:

Is there any way you could split up into several patches?  It's hard to follow when you do multiple things in one patch.  Thanks!

https://github.com/llvm/llvm-project/pull/115212