[analyzer] Fix crash in ObjCGenericsChecker

Fix a crash in the generics checker where DynamicTypePropagation tries
to get the superclass of a root class.

This is a spot-fix for a deeper issue where the checker makes assumptions
that may not hold about subtyping between the symbolically-tracked type of
a value and the compile-time types of a cast on that value.

I've added a TODO to address the underlying issue.

rdar://problem/26086914

llvm-svn: 269227