ObjC migrator: Improve on hueristics.

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