[ObjC] Follow-up r350768 and allow the use of unavailable methods that are

declared in a parent class from within the @implementation context

This commit extends r350768 and allows the use of methods marked as unavailable
that are declared in a parent class/category from within the @implementation of
the class where the method is marked as unavailable.
This allows users to call init that's marked as unavailable even if they don't
define it.

rdar://47134898

Differential Revision: https://reviews.llvm.org/D56816

llvm-svn: 351459