This reverts commit c0564971.

Reason for revert: Speculative revert, ASAN is failing consistently:
https://ci.chromium.org/p/v8/builders/ci/V8%20Win64%20ASAN/15103

Original change's description:
> [parser] Use SmallVector(1) for DeclarationParsingResult::declarations
> 
> 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}

TBR=leszeks@chromium.org,verwaest@chromium.org

Change-Id: I6e46c058f16c965e905f20b8df473a8fb22cc6cc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2419037Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70011}