[clang] [clang][AST] Do not try to handle impossible cases in writeBareSourceLocation (PR #166588)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

Hi, could you please give about more context, i.e. perhaps explain what is going on and why the cases are impossible? It would be great to add this info to the PR description.

https://github.com/llvm/llvm-project/pull/166588