When analyzing functions scopes with the script_scope as parent, don't
skip migrating unresolved variables upwards if we could still be inside
an arrow head, which means accesses to those variables will be
correctly context allocated.

Bug: v8:8510, chromium:1000094
Change-Id: I684f2f8bc692de420203990f93e5c943b5b769c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789705Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63635}