We'll always lookup variables in the entry scope-info-backed chain through a
single entrypoint, hence we can cache any variables we create in that chain on
the entry-point's VariableMap. Otherwise we always have to redo all negative
lookups until we hit the scope that introduces it (the script scope being the
final possible scope to introduce it).

This should allow us to get rid of the ContextSlotCache as well.

Change-Id: I2dc2c9c35d69f35dab3fe3d0353aba1ac68515a5
Reviewed-on: https://chromium-review.googlesource.com/c/1337729Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57542}