[heap] Always promote young objects in Full GC

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}

src/flags/flag-definitions.h

@@ -370,7 +370,7 @@ DEFINE_BOOL(assert_types, false,
 DEFINE_BOOL(allocation_site_pretenuring, true,
             "pretenure with allocation sites")
 DEFINE_BOOL(page_promotion, true, "promote pages based on utilization")
-DEFINE_BOOL(always_promote_young_mc, false,
+DEFINE_BOOL(always_promote_young_mc, true,
             "always promote young objects during mark-compact")
 DEFINE_INT(page_promotion_threshold, 70,
            "min percentage of live bytes on a page to enable fast evacuation")