The cache also needs to keep the code alive. The code objects are
import wrappers and not wasm functions (which we will focus on first),
but eventually we would also like to collect unused import wrappers.
This CL explicitly increments the ref count when {WasmCode} is added
to the cache, and derements all ref counts in the destructor.

R=titzer@chromium.org

Bug: v8:8217
Change-Id: I1bfb276b25b359d83900147e75ec47788e1fa8de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1535825Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60588}