This adds support to ref-count uses of WasmCode, and introduces a
{WasmCodeRefScope} to be used whereever WasmCode objects need to be
kept alive, e.g. because a pointer is passed around.
Future CLs will introduce proper scopes in the whole code base and
enable the DCHECK that's currently commented out.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: I1659a0e9d57cd22fe70e6f2661d0d8af9f0906c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1526005
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60438}