This allows us to reuse AstValueFactory's string table across multiple
parsers, while still releasing memory after each individual parse.

This is mild overkill for all the single parses that don't reuse
AstValueFactories, but there at least the AstRawStrings now end up
grouped together in memory, so that might have mild cache benefits.

Change-Id: I0b378760b601fa4ec6559a0dca5d7ed6f895e992
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3322764Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78338}