QoI: -Wreadonly-iboutlet-property should have the warning's location on the property.

There's no need to refer to the @implementation at all.

Fixes <rdar://problem/13186515>

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@174802 91177308-0d34-0410-b5e6-96231b3b80d8