... into VisitExternalPointer(HeapObject, ExternalPointerSlot,
                              ExternalPointerTag).

Drive-by: introduce ExternalPointerSlot - a slot containing an
ExternalPointer_t value.

This cleanup is a prerequisite for inlining Foreign object fields into
field's holder objects.

Bug: v8:12949
Change-Id: Ifd74ed285796b0952d7d06de82b56c63fd1f7f3e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695361Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81064}