swift234

SILGen: DiverseStack defines stable iterators; let's use them

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