Fixed a few details of method lookup in Objective-C

symbols.  Now we find the correct method.

Unfortunately we don't get the superclass from the
runtime yet so the method doesn't import correctly
(and I added a check to make sure that doesn't hurt
us) but once we get that information right we will
report methods correctly to the parser as well.

Getting superclass information requires a common AST
context for all Objective-C runtime information,
meaning that the superclass and the subclass are in
the same AST context in all cases.  That is the next
thing that needs to be done here.

llvm-svn: 146089