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

[Sergej Salnikov via cfe-commits]

SergejSalnikov wrote:

> Hi, could you please give a bit 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.

PTAL.

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