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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@48970 91177308-0d34-0410-b5e6-96231b3b80d8