ObjectiveC arc[qoi]: When due to change of certain methods'
result type, a diagnostic being issued, issue a 'note' 
mentioning reason behind the unexpected warning.
// rdar://14121570.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186105 91177308-0d34-0410-b5e6-96231b3b80d8