Objective-C: Produce precise diagnostic when
'isa' ivar is accessed provided it is the first
ivar. Fixit hint will follow in another patch.
This is continuation of // rdar://13503456


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178313 91177308-0d34-0410-b5e6-96231b3b80d8