One more fix to store_cached_slot (r=me)
One more fix to store_cached_slot (r=me)

this is the function for coercing & storing into a typed slot where
the type is a plain subclass of Object (not String, int, etc).

null and undefined are allowed first because nullAtoms are not always
kObjectType.  otherwise, for kObjectType values we check that the actual
type is legal for the slot.

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