truncateWithFillerForNewObjectSlotsCount

Slice the current AvailObject into two objects, the left one (at the same starting address as the input), and the right one (a filler object that nobody should ever create a pointer to). The new Filler can have zero post-header slots (i.e., just the header), but the left object must not, since it may turn into an indirection some day and will require at least one slot for the target pointer.