IRGen: If Objective-C interop is disabled foreign classes should use Swift reference counting

I'm not sure how to write an automated test for this yet.

Fixes <rdar://problem/23545207>.

Author: slavapestov

lib/IRGen/GenMeta.cpp

@@ -2941,9 +2941,9 @@ namespace {
     void addValueWitnessTable() {
       ClassDecl *cls = Target;
 
-      auto type = (cls->checkObjCAncestry() != ObjCClassKind::NonObjC)
-        ? CanType(this->IGM.Context.TheUnknownObjectType)
-        : CanType(this->IGM.Context.TheNativeObjectType);
+      auto type = (cls->checkObjCAncestry() != ObjCClassKind::NonObjC
+                   ? this->IGM.Context.TheUnknownObjectType
+                   : this->IGM.Context.TheNativeObjectType);
       auto wtable = this->IGM.getAddrOfValueWitnessTable(type);
       addWord(wtable);
     }
@@ -4743,7 +4743,11 @@ namespace {
     // Visitor methods.
 
     void addValueWitnessTable() {
-      auto type = IGM.Context.TheUnknownObjectType;
+      // Without Objective-C interop, foreign classes must still use
+      // Swift native reference counting.
+      auto type = (IGM.ObjCInterop
+                   ? IGM.Context.TheUnknownObjectType
+                   : IGM.Context.TheNativeObjectType);
       auto wtable = IGM.getAddrOfValueWitnessTable(type);
       addWord(wtable);
     }