[clang] [clang] Fix a dangling reference in clang/utils/TableGen/ClangDiagnosticsEmitter.cpp (PR #119197)

[Haojian Wu via cfe-commits]

hokein wrote:

I'm landing this change now since it is a straightforward use-after-free fix, happy to address any followups.

https://github.com/llvm/llvm-project/pull/119197