[Constraint solver] Dont assume orphans are along in the inactive list.

The inactive list may contain other disjunctions associated with bound
type variables. For now, make sure we recover the orphan directly to
fix the crash in SR-4056 / rdar://problem/30686926. Later, we can
treat these as orphans, too.

(cherry picked from commit 99a1396)

Author: DougGregor

lib/Sema/CSSolver.cpp

@@ -2137,8 +2137,7 @@ bool ConstraintSystem::solveRec(SmallVectorImpl<Solution> &solutions,
       }
     } else {
       // Get the orphaned constraint.
-      assert(InactiveConstraints.size() == 1 && "supposed to be an orphan!");
-      orphaned = &InactiveConstraints.front();
+      orphaned = allOrphanedConstraints[component - firstOrphanedConstraint];
     }
     CG.setOrphanedConstraint(orphaned);
 

test/Constraints/optional.swift

@@ -176,3 +176,21 @@ func rdar29977523(lhs: AnyObject?!, rhs: AnyObject?) {
 
   calleeRdar29977523(lhs, rhs)
 }
+
+// SR-4056
+protocol P1 { }
+
+class C1: P1 { }
+
+protocol P2 {
+    var prop: C1? { get }
+}
+
+class C2 {
+    var p1: P1?
+    var p2: P2?
+
+    var computed: P1? {
+        return p1 ?? p2?.prop
+    }
+}