After https://crrev.com/c/3854501 has landed, we no longer have to
handle the case that we do not find a function's context in the
scope iterator even though the function requires one.

This CL renames `NeedsAndHasContext` to `NeedsContext` since we
always find a scope's context now. Additionally we turn this
assumption into a dedicated check.

R=bmeurer@chromium.org

Bug: chromium:1246907
Change-Id: I6458df76689c0bfa6d6b2f8c421f9ce481855547
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3865153Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82848}