Typically we'll parse a single declaration when parsing variable declarations.
Using on-stack storage rather than std::vector that requires malloc is much
more efficient.

Change-Id: Id99515bb4ce7ea2dae46498f8f9f9d49c33c7353
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2418393
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69995}