swift234

stdlib: lazy FlattenCollection: don't invoke the closure as frequently

FlattenCollection used to evaluate the closure three times per element.
That's acceptable from the technical standpoint, given its lazy
semantics, but not acceptable in practice.  This commit fixes it to only
evaluate it two times when it is converted into an array.

This commit also improves tests.  The new checks used to fail before the
standard library was fixed.

But there are larger issues with FlattenCollection -- it does not model
CollectionType properly.  See next commit.

Swift SVN r30616