Objective-C. Turn off designated initialization warnings on
'init' methods which are unavailable. Subclasses of NSObject
have to implement such methods as a common pattern to prevent
user's own implementation. // rdar://16305460


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203966 91177308-0d34-0410-b5e6-96231b3b80d8