[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).

Radar-Id: rdar://problem/58355212


Signed-off-by: Pierre Habouzit <phabouzit@apple.com>