Objective-C arc: Diagnose when user attempts to
synthesize a property getter method that overrides
a method definition named 'retain' and the like.
Fixes // rdar://13885083


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182039 91177308-0d34-0410-b5e6-96231b3b80d8