[heap] Refactor LinearAllocationArea

Avoid callers operating on raw top/limit where possible and provide
verification of the main invariant.

This is actually related to the refactoring suggest in v8:11958 in
that it cleans up the call sites a bit but doesn't go further than
that.

Bug: v8:11958
Change-Id: I35de29a5cd505b375408fc7c5399f637f3e9c755
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3034741


Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75766}