Flatten cons strings into a buffer instead, as we already did for strings
larger than 64 anyway.

Change-Id: Ida7afb9f7ceb38505d67bedfdfbc43c8b4d8c303
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1599428
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61333}