Address code review comments.

- Make `is_repr_nullable_ptr` freestanding again to avoid usage of
ImproperCTypesVisitor in ClashingExternDeclarations (and don't
accidentally revert the ParamEnv::reveal_all() fix from a week earlier)
- Revise match condition for 1 Adt, 1 primitive
- Generalise check for non-null type so that it would also work for
ranges which exclude any single value (all bits set, for example)
- Make is_repr_nullable_ptr return the representable type instead of
just a boolean, to avoid adding an additional, independent "source of
truth" about the FFI-compatibility of Option-like enums. Also, rename to
`repr_nullable_ptr`.

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