[DiagnosticVerifier] Add -verify-ignore-unrelated flag

This adds the implementation required for later changing the default
behaviour of the -verify flag to error when diagnostics are emitted
in buffers other than the main file and files added with
-verify-additional-file. To keep the current behaviour, use the flag
-verify-ignore-unrelated. This flag is added as a no-op so that tests
can start using it before the new behaviour is enabled by default.

lib/Frontend/CompilerInvocation.cpp

@@ -2588,6 +2588,7 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_verify_apply_fixes))
     Opts.VerifyMode = DiagnosticOptions::VerifyAndApplyFixes;
   Opts.VerifyIgnoreUnknown |= Args.hasArg(OPT_verify_ignore_unknown);
+  Opts.VerifyIgnoreUnrelated |= Args.hasArg(OPT_verify_ignore_unrelated);
   Opts.SkipDiagnosticPasses |= Args.hasArg(OPT_disable_diagnostic_passes);
   Opts.ShowDiagnosticsAfterFatalError |=
     Args.hasArg(OPT_show_diagnostics_after_fatal);