SE-0157: Enable recursive protocol constraints by default.

We don't know if they work in general, but specific cases of recursive
protocol constraints are working, and enabling them doesn't break
anything.

lib/Frontend/CompilerInvocation.cpp

@@ -1007,8 +1007,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   }
   
   Opts.EnableASTScopeLookup |= Args.hasArg(OPT_enable_astscope_lookup);
-  Opts.EnableRecursiveConstraints |=
-    Args.hasArg(OPT_enable_recursive_constraints);
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
   Opts.EnableConstraintPropagation |= Args.hasArg(OPT_propagate_constraints);
   Opts.IterativeTypeChecker |= Args.hasArg(OPT_iterative_type_checker);