ObjectiveC. Allow readonly properties without an explicit ownership
(assign/unsafe_unretained/weak/retain/strong/copy) in super class
to be overridden by a property with any explicit ownership in the 
subclass. // rdar://15014468


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191971 91177308-0d34-0410-b5e6-96231b3b80d8