Add -print-diagnostic-groups flag

This change adds the `-print-diagnostic-groups` flag as described by SE-0443.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -961,7 +961,7 @@ public:
         // Emit a specific unavailable message when we know why a decl can't be
         // exposed, or a generic message otherwise.
         auto diagString = M.getASTContext().Diags.diagnosticStringFor(
-            diag.getID(), /*PrintDiagnosticNames=*/false);
+            diag.getID(), PrintDiagnosticNamesMode::None);
         DiagnosticEngine::formatDiagnosticText(os, diagString, diag.getArgs(),
                                                DiagnosticFormatOptions());
         os << "\");\n";