Objective-C: Issue warning in couple of obscure cases

when property autosynthesis does not synthesize a property.
When property is declared 'readonly' in a super class and
is redeclared 'readwrite' in a subclass. When a property
autosynthesis causes it to share 'ivar' with another property.
// rdar://13388503


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176889 91177308-0d34-0410-b5e6-96231b3b80d8