swift234

[Constraint System] Kill the ArchetypeOpener; bind type variables instead. NFC

The ArchetypeOpener was used only to replace dependent types with
archetypes (or concrete types) within the opening context. We can do
the same simply by letting the constraint system create type variables
and then binding those type variables to the appropriate
archetypes/concrete types in that context.

Eliminate the two DependentTypeOpener entry points that were only used
by the ArchetypeOpener.