IntelliSense should not show myself when in base class/interface context

[davkean]

We shouldn't offer to implement/derive from ourselves:

[Pilchie]

We should also remove: Enums, structs, and delegate types from the list here.

[CyrusNajmabadi]

We shouldn't offer to implement/derive from ourselves:

Note: it's legal to write:

class C : C.Inner
{
    public interface Inner
    {

    }
}

We could consider filtering these out if there aren't available inner types the user might be referencing.

[CyrusNajmabadi]

@DoctorKrolic were you working on fixing this?