When looking for overridden ObjC methods, don't ignore 'hidden' ones.

When using modules we should not ignore overridden methods from
categories that are hidden because the module is not visible.
This will give more consistent results (when imports change) and it's more
correct since the methods are indeed overridden even if they are not "visible"
for lookup purposes.

rdar://13350796

llvm-svn: 178374