Add a frontend option to enable parsing "operator protocol designators".

lib/Frontend/CompilerInvocation.cpp

@@ -161,6 +161,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalPropertyBehaviors |=
     Args.hasArg(OPT_enable_experimental_property_behaviors);
 
+  Opts.EnableOperatorDesignatedProtocols |=
+      Args.hasArg(OPT_enable_operator_designated_protocols);
+
   if (auto A = Args.getLastArg(OPT_enable_deserialization_recovery,
                                OPT_disable_deserialization_recovery)) {
     Opts.EnableDeserializationRecovery