DeferCleanup pushes a new temporary cleanup to catch non-local returns
from the defer block, so we have to use stable iterators while emitting
cleanups.
There's no good deterministic test case for this -- it would manifest
as memory corruption if the underlying storage of the DiverseStack
grew beyond the inline storage. Add a reduced version of the original
user-reported test case that triggers it reliably -- I had a hard time
coming up with anything simpler.
Fixes <rdar://problem/21437203>.
Swift SVN r29658
