[clang] [clang][deps] Ensure DiagnosticConsumer::finish is always called (PR #127110)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

This makes sense in principle, but I'm wondering whether https://github.com/llvm/llvm-project/pull/100681 is the better approach. Seems like having the client call `finish()` is simpler than doing that "automatically" in `CompilerInstance::ExecuteAction()` and then trying to catch all the cases where that was not reached.

https://github.com/llvm/llvm-project/pull/127110