ObjC migrator: Improve on hueristics.
migrate to 'copy attribute if Object
class implements NSCopying otherwise 
assume implied 'strong'. Remove 
lifetime qualifier on property as it has
moved to property's attribute. Added TODO
comment for future work by poking into
setter implementation.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186037 91177308-0d34-0410-b5e6-96231b3b80d8