[clang] [clang] Don't fail `clang::ExecuteCompilerInvocation()` for unrelated errors (PR #158695)

[Steven Wu via cfe-commits]

https://github.com/cachemeifyoucan commented:

Can you add more comment to explain when do we actually call this function when Diagnostics Engine already has error? This will help to evaluate what is a right thing to do.

https://github.com/llvm/llvm-project/pull/158695