swift234

Add GenericSignature APIs to query the requirements on dependent types.

Extend GenericSignature to be able to answer queries about the
requirements placed on dependent types, e.g, are the class-bound, to
what protocols must they conform, etc. Implement this using a
lazily-created ArchetypeBuilder on the canonical generic signature.

NFC and as-yet-untested; this is staging for reducing our dependence
on the "all archetypes" list.

Swift SVN r32340