Allow caching the result of allocating AstConsStrings, to allow
sharing of inferred names between functions.

This is a partial revert of https://crrev.com/c/2020953, with
the observation that *some* AstConsStrings are always flattened,
while others are only ever used as ConsStrings, so we want to
allow the allocation to be lazy while still caching the result.

As a drive-by, cleanup the old AstConsString linked list fields.

Bug: chromium:1011762
Bug: chromium:1048082
Change-Id: Icc14342eb3f6f97359596b42b2c296cbc49fd791
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2042093
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66168}