[DiagnosticVerifier] Enable -verify-ignore-unrelated

This enables the previously added -verify-ignore-unrelated flag. When
-verify is used without -verify-ignore-unrelated, diagnostics emitted in
buffers other than the main file and those passed with
-verify-additional-file (except diagnostics emitted at <unknown>:0) will
now result in an error. They were previously ignored. The old behaviour
is still available as opt-in using -verify-ignore-unrelated.

Author: hnrklssn

lib/Frontend/DiagnosticVerifier.cpp

@@ -1561,6 +1561,11 @@ bool DiagnosticVerifier::finishProcessing() {
     // For <unknown>, all errors are unexpected.
     Result.HadUnexpectedDiag |= HadError;
   }
+  if (!IgnoreUnrelated) {
+    bool HadError = verifyUnrelated(CapturedDiagnostics);
+    Result.HadError |= HadError;
+    Result.HadUnexpectedDiag |= HadError;
+  }
 
   if (Result.HadUnexpectedDiag)
     printRemainingDiagnostics();