This generalizes the existing support for Map and Set iteration in the
CSA a bit and makes it possible to reuse the logic to implement forEach
as well. It also introduces an empty_ordered_hash_table, which is used
as a sentinel for exhausted iterators to avoid the need to deal with
undefined there as well (not observable from JavaScript).

TBR=ulan@chromium.org
R=jgruber@chromium.org

Bug: v8:5269, v8:5717
Change-Id: Ifb9ec5ecb20939aa9b7d2471537f8ccd4af04c8f
Reviewed-on: https://chromium-review.googlesource.com/565260Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46547}