[DiagnosticEngine] Mark module import diagnostic errors as 'fatal' errors, and supress subsequent diagnostics

to avoid a cascade of errors that drown out the original failure.

Swift SVN r17141

lib/Frontend/CompilerInvocation.cpp

@@ -621,6 +621,10 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
     Opts.SkipDiagnosticPasses = true;
   }
 
+  if (Args.hasArg(OPT_show_diagnostics_after_fatal)) {
+    Opts.ShowDiagnosticsAfterFatalError = true;
+  }
+
   return false;
 }