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.

This speculatively re-applies 7576a9100976c3de5c8d8c22ba5be7d2410eb905,
i.e. reverts commit 11ab3d537f96b05742bba20c2dbd021c3d0370f4.
We have not been able to duplicate the build failure in
independent testing; it might have been spurious or unrelated.