We can reuse part of ConcurrentLookupIterator::TryGetOwnConstantElement
to read a char from a string concurrently.

Bug: v8:7790, v8:11012
Change-Id: Iaa75e0cdb457963e89e6bbbdb79766502286cc2d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097277
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76318}