[clang] [NFC][Clang] Use StringRef instead of string in ClangDiagnosticEmitter (PR #115959)

[Erich Keane via cfe-commits]

erichkeane wrote:

FWIW, LGTM, I'm Ok with it as-is, but let @kazutakahirata do the approval when he's happy.

https://github.com/llvm/llvm-project/pull/115959