This adds the {CurrentGCInfo} data structure to the wasm engine. It
holds all information needed for the current GC cycle, which is
currently only the set of Isolates that still need to report their live
code, and the set of dead wasm code (which is potentially reduced when
Isolates report live code).

Running the GC is guarded by the new '--wasm-code-gc' flag. I will add
this to the --future variant in a follow-up CL.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I82e96d986cf5a758bc0f94e49e13ad78fae4e935
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1559738
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60721}