By introducing a globally known map for each generic type.
These maps are never used to allocate objects, they only
serve as sentinels for generic heap types.

Bug: v8:7748
Change-Id: I950a8c712dc1510759a833fe9122b9e9a6222dc2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2288860
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68755}