swift234

Pick the potential archetype we use for the actual archetype more intelligently.

Archetypes either need to be a root, which corresponds to a type
parameter, or need to be parented by another archetype. When the
representative of an equivalence class of potential archetypes has a
parent, but that parent has been made equivalent to some concrete type
via a same-type constraint, choose a different potential archetype as
the "anchor", i.e., one that has no concrete types in its parent
chain. Fixes rdar://problem/21625478 and the canonical example in
rdar://problem/17619740.

Swift SVN r29898