[objc] If we don't know for sure what the designated initializers of the superclass are,
assume that a [super init..] inside a designated initializer also refers to a designated one
and do not warn.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197202 91177308-0d34-0410-b5e6-96231b3b80d8