This makes usages less verbose, and is consistent with other existing
enums.
Also, we can use brace initialization to avoid boilerplate when creating
a DynamicTiering value.

Drive-by: Rename a 'kIncludeLiftoff' variable to 'include_liftoff'
because it is not a static constant.

R=jkummerow@chromium.org

Bug: v8:12281
Change-Id: Ie45fdb550241a8b9ca4e2a31b7c27500939fa247
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3585566Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79993}