I ran into this same issue, although in my case the app is already not using ARC. I added the following to the "Other Liinker Flags" setting:
-Xlinker -U -Xlinker _objc_loadClassref
I'm not sure this is a proper fix, but at least it gets the link to succeed and does not seem to cause any issues during run time. I'm using this just so I can get some work done, not necessarily as a final fix. I fear that leaving this in place might break the app on older OSs.
Anyway, maybe this can get you back to a working build, at least until someone from Apple gives you a better answer...