Divide MarkExpressionAsAssigned into MarkExpressionAsAssigned and
MarkPatternAsAssigned so it's clear when we need to mark just a single variable
or an entire destructuring assignment pattern.

Change-Id: Ia188b3d9b15944a1859676f483df229225ce8404
Reviewed-on: https://chromium-review.googlesource.com/c/1270945
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56505}