[Runtime] Reference ObjC class objects indirectly in conformance records.

Within conformance records, reference Objective-C class objects
indirectly so the runtime can update those references appropriately.
We don't need to do this for classes with Swift metadata.

Make all OBJC_CLASS_REF symbols object-local using "\01l", which
prevents the linker from producing incorrect relative addresses.

Fixes the ABI-affecting part of rdar://problem/36310179.

include/swift/IRGen/Linking.h

@@ -766,6 +766,9 @@ public:
   bool isForeignTypeMetadataCandidate() const {
     return getKind() == Kind::ForeignTypeMetadataCandidate;
   }
+  bool isObjCClassRef() const {
+    return getKind() == Kind::ObjCClassRef;
+  }
 
   /// Determine whether this entity will be weak-imported.
   bool isWeakImported(ModuleDecl *module) const {