This ensures that the {Code::builtin_index} field is only set during
allocation of new {Code} objects, making this field truly immutable.

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

Change-Id: Ic793346976183149e2d077e92cb9da3c925ea865
Reviewed-on: https://chromium-review.googlesource.com/774439Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49414}