Objective-C migrator: when inferring 'readonly' property of an
Objective-C object conforming to 'NSCopying' protocol, infer
a 'copy' property, instead of 'strong'. // rdar://15525937


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197102 91177308-0d34-0410-b5e6-96231b3b80d8