[NFC] Fix issues with -verify mode

This commit fixes two weird bugs in -verify mode:

1. SourceLocs from the wrong SourceManager could be passed through a ForwardingDiagnosticConsumer into the DiagnosticVerifier.

2. -verify-additional-file did not error out correctly when the file couldn’t be opened.

No tests, as we only have basic tests for the diagnostic verifier.

lib/Frontend/Frontend.cpp

@@ -307,6 +307,7 @@ bool CompilerInstance::setupDiagnosticVerifierIfNeeded() {
         Diagnostics.diagnose(SourceLoc(), diag::error_open_input_file,
                              filename, result.getError().message());
         hadError |= true;
+        continue;
       }
 
       auto bufferID = SourceMgr.addNewSourceBuffer(std::move(result.get()));