objective-c: Using existing infrastructure for finding

overridden  methods to diagnose their type mismatch.
This is a general solution for previous fixes
for // rdar://6191214 and // rdar://9352731
and removes lots of duplicate code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137222 91177308-0d34-0410-b5e6-96231b3b80d8