swift234

Start using the forced collection downcast entry points.

This means that we'll get deferred checking of array and dictionary
downcasts when writing "arr as Derived[]", 
"(dict as? Dictionary<DerivedKey, DerivedValue>)!", etc, when the
collection can do so.

This is both a general optimization and also staging for
<rdar://problem/17319154>.


Swift SVN r18975