Decommit region tails (#66008)

Decommit region tails (#66008)

I observed that with gen 1 regions, we often get into the situation that gen 1 is much smaller per heap than a region. So it makes sense to decommit the tail end of the last region in an ephemeral generation guided by the budget for that generation.

To implement this, I reactivated decommit_target for regions and have decommit_step call decommit_ephemeral_segment_pages_step which in the regions case needs to synchronize with the allocator. This is done by taking the more space lock.

Note that with default settings, this decommitting logic will usually only apply to gen 1 because normally gen 0 is larger than a region. It can still happen for gen 0 though if gen 0 has pins and thus already has enough space to satisfy the budget. Then we will decommit the tail end of the last region in gen 0.