In a subsequent CL, I'll need to do String allocations in Turbofan (in
the background), where only a LocalFactory is available. By moving
those string allocation functions to FactoryBase, they will also be
available in the LocalFactory.

Change-Id: I066bbd4b5016645de183633ef237986e0ae50f5d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3811581Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82262}