[Constraint solver] Only build disjunctions when they're unsolved.

Author: DougGregor

lib/Sema/ConstraintSystem.cpp

@@ -1389,6 +1389,12 @@ void ConstraintSystem::addOverloadSet(Type boundType,
                                       OverloadChoice *favoredChoice) {
   assert(!choices.empty() && "Empty overload set");
 
+  // If there is a single choice, add the bind overload directly.
+  if (choices.size() == 1) {
+    addBindOverloadConstraint(boundType, choices.front(), locator);
+    return;
+  }
+
   SmallVector<Constraint *, 4> overloads;
 
   // As we do for other favored constraints, if a favored overload has been

lib/Sema/ConstraintSystem.h

@@ -1490,7 +1490,7 @@ class ConstraintSystem {
     if (isFavored)
       constraint->setFavored();
 
-    addConstraint(constraint);
+    addUnsolvedConstraint(constraint);
   }
 
   /// Whether we should add a new constraint to capture a failure.