This fast path copies the backing store and replaces holes with undefined.
In the case where the array is holey but there is no actual holes, the
resulting array is of the same elements kind as the source array. If a hole
does appear, the resulting array will be of PACKED_ELEMENTS kind so that it
can contain undefined.

The builtin CloneFastJSArrayFillingHoles includes this fast path, but
CloneFastJSArray does not (it still behaves as before). In case of fast
packed arrays, CloneFastJSArrayFillingHoles behaves the same as
CloneFastJSArray.

Bug: chromium:881273, v8:7980
Change-Id: I49c641c1a673313f06aeed93077031ab6b017b6d
Reviewed-on: https://chromium-review.googlesource.com/1236573
Commit-Queue: Hai Dang <dhai@google.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56209}