[clang] [Clang][diagtool] Fix memory leak in ShowEnabledWarnings (PR #191711)

[Naveen Seth Hanig via cfe-commits]

https://github.com/naveen-seth approved this pull request.

Thanks for the fix!

(I am not sure if we still need the `TextDiagnosticBuffer` and will investigate this soon. But I think this can definitely land directly.)

https://github.com/llvm/llvm-project/pull/191711