[objc_direct] Small updates to help with adoption.
Add fixits for messaging self in MRR or using super, as the intent is
clear, and it turns out people do that a lot more than expected.
Allow for objc_direct_members on main interfaces, it's extremely useful
for internal only classes, and proves to be quite annoying for adoption.
Add some better warnings around properties direct/non-direct clashes (it
was done for methods but properties were a miss).
Add some errors when direct properties are marked @dynamic.
Signed-off-by: Pierre Habouzit <firstname.lastname@example.org>
Differential Revision: https://reviews.llvm.org/D73755