Improve StressHeap

[Maoni0]

this is related to LocalGC project dotnet/coreclr#10391.

StressHeap should really be on the EE side but currently it uses make_unused_array to make free objects in existing objects in hope of making them move. The problem is these objects are likely already in gen2 and unless you do a gen2 GC they are not going to move anyway. The way folks use GCStress does tend to suggest that we do a lot of gen2 GCs but we can make this more efficient by simply having GC allocate free objects in gen0 which has a much bigger chance of making these objects move. We also wouldn't need to expose something like make_unused_array on the interface.

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