This is a step towards reducing the number of StringTableKeys to the absolute
minimum so we can better optimize how they work. This always flattens
ConsStrings into a buffer to avoid expensive comparison with cons string (as
well as hash computation).

Change-Id: I6dcf0bdd2a722f490dad02b7f887083e1ac46000
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598707Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61289}