[Custom Descriptors] Use placeholder describees in GTO #7888
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GTO tries to optimize out unused descriptor types, but sometimes cannot because the descriptor type must remain a descriptor to be a valid supertype of a subtype that will remain a descriptor. To optimize the original described type to not have a descriptor while simultaneously keeping the descriptor a valid supertype, insert a new, trivial placeholder type for the descriptor to describe. Since this new type is not otherwise used in the module, it should be able to be optimized out after subsequent rounds of unsubtyping and other optimizations.
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GTO tries to optimize out unused descriptor types, but sometimes cannot
because the descriptor type must remain a descriptor to be a valid
supertype of a subtype that will remain a descriptor. To optimize the
original described type to not have a descriptor while simultaneously
keeping the descriptor a valid supertype, insert a new, trivial
placeholder type for the descriptor to describe. Since this new type is
not otherwise used in the module, it should be able to be optimized out
after subsequent rounds of unsubtyping and other optimizations.