[ConstraintSystem] Add a staging option for the operator protocols work.

Add a staging option for enabling upcoming changes in the solver in
support of using the protocols designated in Policy.swift to guide
operator type checking.

lib/Frontend/CompilerInvocation.cpp

@@ -164,6 +164,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableOperatorDesignatedProtocols |=
       Args.hasArg(OPT_enable_operator_designated_protocols);
 
+  Opts.SolverEnableOperatorDesignatedProtocols |=
+      Args.hasArg(OPT_solver_enable_operator_designated_protocols);
+
   if (auto A = Args.getLastArg(OPT_enable_deserialization_recovery,
                                OPT_disable_deserialization_recovery)) {
     Opts.EnableDeserializationRecovery