some cleanups on top of David's patch. There are still two

remaining open issues I've communicated to him:

1) self can be assigned to, and his patch didn't handle it correctly.
2) CollectObjCIvarTypes is N^2 (because each subclass reprocesses
   all parent class ivars) and flattens classes.  If A derives from B, 
   and both have an int, I'd expect to get { {i32}, i32}, not { i32, i32}.

David, please review.

llvm-svn: 48970