Revert r139989 and r140031, which implemented the Objective-C type

system change in <rdar://problem/10109725> that allows conversion from
'self' in class methods to the root of the class's hierarchy. This
conversion rule is a hack that has non-trivial repurcussions
(particularly with overload resolution).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140605 91177308-0d34-0410-b5e6-96231b3b80d8