Perform a dynamic method call if a class has objc ancestry in speculative devirt as fallback.

If a class has an @objc ancestry this class can be dynamically overridden and
therefore we don't know the default case even if we see the full class
hierarchy.

rdar://23228386

lib/SILOptimizer/Transforms/SpeculativeDevirtualizer.cpp

@@ -217,6 +217,13 @@ static bool isDefaultCaseKnown(ClassHierarchyAnalysis *CHA,
   if (CD->isFinal())
     return true;
 
+  // If the class has an @objc ancestry it can be dynamically subclassed and we
+  // can't therefore statically know the default case.
+  auto Ancestry = CD->checkObjCAncestry();
+  if (Ancestry != ObjCClassKind::NonObjC &&
+      Ancestry != ObjCClassKind::ObjCMembers)
+    return false;
+
   // Without an associated context we cannot perform any
   // access-based optimizations.
   if (!DC)