Subclasses can now access it via {code()}, even in constexpr contexts.

R=tebbi@chromium.org

Bug: v8:9810
Change-Id: I3cc6872f568f38db8cdbcda69ac0e203f839cda5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1914216Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64962}