This move the {Code::next_code_link} into the data container, making it
possible to mutate the field even when {Code} objects are protected. It
also introduces a dedicated body descriptor for the container allowing
for tagged fields to be part of the container.

R=ulan@chromium.org
BUG=v8:6792

Change-Id: I56a9d53e8bb35aeb0a7036e3abf3ebee1ba2928d
Reviewed-on: https://chromium-review.googlesource.com/738184
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48966}