Always add lambda to scope chain to eliminate branch in eval_scope

We convert all other scopes to lambda's so it makes sense that we should
always returns a lambda on a ThroughReflection as well. This eliminates
the need to check if the scope is a Relation.
Merge pull request #16801 from eileencodes/refactor-scope_chain-on-through-refelction-to-eliminate-branch-in-eval_scope

Always add lambda to scope chain to eliminate branch in eval_scope