objective-C: don't issue bogus warning about
"auto-synthesized may not work correctly with 'nib' loader"
when 'readonly' property is redeclared 'readwrite' in class
extension. // rdar://13123861


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174775 91177308-0d34-0410-b5e6-96231b3b80d8