Compacting GC: Handle black holes in large objects.

Andreas Klebingerrequested to merge
wip/andreask/compacting_gc_crash into master 
Compacting GC: Handle black holes in large objects.

As #14497 showed black holes can appear inside large objects when
we capture a computation and later blackhole it like we do for AP_STACK
closures.

Fixes #24791

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