To use members of protocol extensions on existential types, we
introduce an OpenExistentialExpr expression to open up the existential
type (into a local archetype) and perform the operations on that local
archetype.
Unlike with uses of initializers or dynamic-Self-producing
methods of protocols, which produce similar ASTs, we have the type
checker perform the "open" operation and then track it through
constraint application. This scheme is better (because it's more
direct), but it's still using a simplistic approach to deciding where
the actual OpenExistentialExpr goes that needs improvement.
Swift SVN r26964
