The various 'isExplicit' checks were wrong -- they should apply
to protocol requirements only, not protocol extension members.
Also, we weren't performing this check for type aliases in
protocols, so you would get a misleading diagnostic telling you
the type alias must be declared private/internal, when in fact
type aliases in protocols cannot have their own access control.
Finally, the "should be declared <X>" diagnostics (the
!isExplicit case) need to state the most visible access level
that will work, and in a few cases we would say private instead
of fileprivate here.
