Re-land of https://crrev.com/cf13dda1ba25e8293ea143f33c6c5f6233a39c86,
fixing the issue with vector stores.

Class methods always have the class scope on their scope chain in order
to implement strong mode checks. Previously, that scope wasn't attached
to the ClassLiteral for anonymous classes (since the scope contained
no bindings).

This patch simply puts that same scope on the ClassLiteral, anonymous
or not, which simplifies other code that needs to reason about the scope
of a class and its methods.

Review URL: https://codereview.chromium.org/1418433002

Cr-Commit-Position: refs/heads/master@{#31381}