Lazy deserialization needs to determine the underlying builtin by looking at
the SharedFunctionInfo.

This packs the builtin_id into the SFI::function_data field, and adds
convenience functions to Code as a drive-by addition.

Bug: v8:6624
Change-Id: I59093815aa6937342302153ebc95dd60edb0064e
Reviewed-on: https://chromium-review.googlesource.com/641490
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47712}