Emit an early error when BindingPatterns are used in a VariableDeclaration
or LexicalBinding without an Initializer.

BUG=v8:4532
LOG=N
R=adamk@chromium.org, rossberg@chromium.org, wingo@igalia.com

Review URL: https://codereview.chromium.org/1416753009

Cr-Commit-Position: refs/heads/master@{#31802}

Add() becomes AddLocalExport, which takes an export_name and a local_name.
New parsing tests exercise this.

Also start generating exports for default exports (though this doesn't yet
handle anonymous default exports).

BUG=v8:1569
LOG=n

Review URL: https://codereview.chromium.org/934323004

Cr-Commit-Position: refs/heads/master@{#26758}