objective-c: warn on use of property setters
backing two propeties because proprty names
match except for first letter being of different
case. // rdar://11528439, [PR12936].


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157435 91177308-0d34-0410-b5e6-96231b3b80d8