swift234

Allow lazy resolution of nested archetypes within the archetype builder.

While the archetype builder is constructing archetypes, it eagerly
produces the set of nested archetypes for each archetype. If the
construction of any of those nested archetypes refers to another
archetype, we would fail unceremonerously with an assertion. Fill in
the nested type names (but not their actual types) eagerly, allowing
the actual types to be determined lazily.

This is plumbing for rdar://problem/21620908, which still trips an
unrelated assertion, preventing a useful testcase at this step.

Swift SVN r29841