[objcmt] Check for classes that accept 'objectForKey:' (or the other selectors

that the migrator handles) but return their instances as 'id', resulting
in the compiler resolving 'objectForKey:' as the method from NSDictionary.

When checking if we can convert to subscripting syntax, check whether
the receiver is a result of a class method from a hardcoded list of
such classes. In such a case return the specific class as the interface
of the receiver.

llvm-svn: 159788