objc - don't complain about unimplemented property when conforming

protocol declares the property, as well as one of its superclasses.
Property will be implemented in the super class. // rdar://10120691


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140586 91177308-0d34-0410-b5e6-96231b3b80d8