Sema: Avoid asking for the SecondType of Constraints that don't have them.

Assertions tripped while trying to reproduce SR-5513.

Author: jckarter

lib/Sema/CSDiag.cpp

@@ -2833,6 +2833,11 @@ bool FailureDiagnosis::diagnoseGeneralConversionFailure(Constraint *constraint){
     fromType = CS.getType(sub);
   }
 
+  // Bail on constraints that don't relate two types.
+  if (constraint->getKind() == ConstraintKind::Disjunction
+      || constraint->getKind() == ConstraintKind::BindOverload)
+    return false;
+
   fromType = fromType->getRValueType();
   auto toType =
       CS.simplifyType(constraint->getSecondType())->getWithoutImmediateLabel();

lib/Sema/ConstraintGraph.cpp

@@ -641,7 +641,6 @@ static bool shouldContractEdge(ConstraintKind kind) {
   case ConstraintKind::BindParam:
   case ConstraintKind::BindToPointerType:
   case ConstraintKind::Equal:
-  case ConstraintKind::BindOverload:
 
   // We currently only allow subtype contractions for the purpose of 
   // parameter binding constraints.