Remove AstConsString "internalization", and instead make the conversion
to heap String be on-demand with an Allocate method. We never actually
need the heapified cons string more than once, so there's no need to do
the internalization walk or do the next/string union dance in the
AstConsString class.

This also allows us to specify how we want to allocate the String at the
call site. In particular, it allows us to allocate a flat SeqString rather
rather than a ConsString. This allows us to avoid allocating ConsStrings
which will just be passed to a flatten call, and especially avoid
allocating dead ConsStrings in the off-thread old space.

Bug: chromium:1011762
Bug: chromium:1043168
Change-Id: Id851f2f7529d92ad7e5388eb22823fd6d1959cd0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2020953Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66042}