Fix overload set construction to not have an extra element in the disjunction.

Any time we had a favored choice for a disjunction, we attempted to
not add it twice, but that little optimization was faulty.

This should speed up some expression type checking a tiny bit in cases
where the favored choice did not result in a solution.

Noticed by observation.

Author: rudkx

lib/Sema/ConstraintSystem.cpp

@@ -1433,7 +1433,7 @@ void ConstraintSystem::addOverloadSet(Type boundType,
     overloads.push_back(bindOverloadConstraint);
   }
 
-  for (auto choice : choices) {
+  for (auto &choice : choices) {
     if (favoredChoice && (favoredChoice == &choice))
       continue;