These will never work properly because of phase ordering issues with
the current declaration checker design. Since we can always express
the same thing with the protocol inheritance clause instead, just
diagnose this as an error instead of trying to hack around it.
Fixes <rdar://problem/38077232>, <https://bugs.swift.org/browse/SR-5581>.
Whenever we form a potential archetype that is unresolved (because it
names a member wasn't known at the time the potential archetype was
formed), create a corresponding delayed requirement to resolve the
potential archetype. This ensures that all potential archetypes get a
chance to be resolve, fixing the
nested type should have matched associated type
assertion in rdar://problem/31401161 (and others).
