This only tackles the protocol case (<rdar://problem/17510790>); it
does not yet generalize to an arbitrary "class" requirement on either
existentials or generics.
Swift SVN r19896
When projecting an existential into an opened archetype, bind the
archetype with metadata and witness tables extracted from the
existential. Tweak SILGen so that it doesn't destroy the opened
archetype value an extra two times.
Use an executable testcase to ensure end-to-end operation, because we
still don't have a parsable form existential projection to opened
archetype instructions.
Swift SVN r13755
