swift234

Sema: Don't set type of AssociatedTypeDecl until we have an archetype

Otherwise the verifier can crash because hasType() returns true but
getType() gives us a MetatypeType that hits a null pointer in
desugaring.

The computeType() calls appear in a few too many places for my liking;
would be nice to clean this up further or replace everything with
interface types one day.

Fixes <rdar://problem/19606899>.

Swift SVN r30388