swift234

Include access functions for the metadata and witness tables
of associated types in protocol witness tables.

We use the global access functions when the result isn't
dependent, and a simple accessor when the result can be cheaply
recovered from the conforming metadata.  Otherwise, we add a
cache slot to a private section of the witness table, forcing
an instantiation per conformance.  Like generic type metadata,
concrete instantiations of generic conformances are memoized.

There's a fair amount of code in this patch that can't be
dynamically tested at the moment because of the widespread
reliance on recursive expansion of archetypes / dependent
types.  That's something we're now theoretically in a position
to change, and as we do so, we'll test more of this code.