Enable --always-promote-young-mc by default. This enforces that the
young generation is empty after a full GC to simplify the implementation of
some features.

For example array buffer sweeping already assumes that this flag is
enabled.

Bug: v8:10064
Change-Id: I9d3873b9eb7bec4de897c5c95bdc514d165265d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2051943Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66236}