We currently only expose this to desugarings and not in the grammar
to keep 'const' and 'let' bindings consistent.
A side-effect of this change is that it is now possible to use a
shadowed name in the initializer of a const binding.

Bug: v8:7793
Change-Id: Ic2ca6af0735acf0e748d394f9039fe6612bd4a06
Reviewed-on: https://chromium-review.googlesource.com/1150534
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54755}