ObjectiveC migrator: For consistency, also infer
'instancetype' for known family of methods
with related result type; such as 'init'
methods. // rdar://14987948


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190956 91177308-0d34-0410-b5e6-96231b3b80d8