[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.

lib/IRGen/Linking.cpp

@@ -191,7 +191,7 @@ std::string LinkEntity::mangleAsString() const {
     case Kind::ObjCClassRef: {
       llvm::SmallString<64> tempBuffer;
       StringRef name = cast<ClassDecl>(getDecl())->getObjCRuntimeName(tempBuffer);
-      std::string Result("OBJC_CLASS_REF_$_");
+      std::string Result("\01l_OBJC_CLASS_REF_$_");
       Result.append(name.data(), name.size());
       return Result;
     }