This allows us to assert at compile time that a class instance is
assigned, which is particularly useful for Guard classes.

Change-Id: Id16b2bb70d29573566e821c908c1169d49ec57af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2552415
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71397}