let f = function g() { ... } declares "g" inside the function. This
CL makes the preparser declare it too, and saves + restores the scope data for
it.

BUG=v8:5516

Change-Id: Id4c64f446d30f5252038cfb0f0f473b85ba24a9b
Reviewed-on: https://chromium-review.googlesource.com/544816
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46133}