[SemaObjC] For the semantics of the designated-initializer attribute, consider that the ObjC interface of the class
includes its class extensions.

This is follow-up for rdar://16305347

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205065 91177308-0d34-0410-b5e6-96231b3b80d8