This guarantees that if it's context-allocated, it'll be the first
slot in the context. That in turn allows us to drop a special index on
scope-info pointing at the receiver entry; once we update arguments
object handling to take the receiver possibly being there into
account.

Change-Id: Idfd06cf172e6905b02c8d17a962382e2a9ea0874
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3211999
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77360}