[Sema][ObjC] Allow silencing -Wobjc-designated-initializers warnings by

declaring an unavailable method in the subclass's extension that
overrides the designated initializer in the base class.

r243676 made changes to allow declaring the unavailable method in the
subclass interface to silence the warning. This commit additionally
allows declaring the unavailable method in the class extension.

rdar://problem/42731306

llvm-svn: 355175