[Constraint graph] Don't drop unidirectional constraints.

Ye olde "break should have been continue" bug. This undoubtedly has
problems elsewhere, but I don't have a specific case until the next
commit.

Author: DougGregor

lib/Sema/ConstraintGraph.cpp

@@ -942,7 +942,7 @@ namespace {
         for (auto lhsTypeRep : lhsTypeReps) {
           for (auto rhsTypeRep : rhsTypeReps) {
             if (lhsTypeRep == rhsTypeRep)
-              break;
+              continue;
 
             insertIfUnique(oneWayDigraph[rhsTypeRep].outAdjacencies,lhsTypeRep);
             insertIfUnique(oneWayDigraph[lhsTypeRep].inAdjacencies,rhsTypeRep);