Rather than allocating one in the zone and swapping them on discardable zone
swap, we simply swap the zone in the inferrer and allow the stacks to grow in
the outer zone. The inner segments will be dropped anyway.

This also introduces a PreParserFuncNameInferrer that just has dummy
implementations. That way we can avoid checking whether fni_ is nullptr at
runtime.

Change-Id: I0ff41b16d31571fc4606fd46b705d80b423343eb
Reviewed-on: https://chromium-review.googlesource.com/1238573Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56132}