With this change, the GC will compute the size for ScopeInfo instances
based on a combination of flags, context_local_count, and possibly
module_variable_count, rather than using the FixedArray-style length
field. After this change and a few more cleanups, we should be able to
remove that length field and save a few bytes.

Bug: v8:8952
Change-Id: Ica8e51ee106685b44fcc55556b4bb124afc91cfa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2598461
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72231}