`empty_enums` doesn't account for how the type is used

Description

Empty enums are used as an existential type and in the typestate pattern. These uses need the nominal type and cannot simply be aliases of !.

Typestate example:

enum State1 {}
enum State2 {}

struct Machine<S>(PhantomData<S>);
impl Machine<State1> {
  fn do1(self) -> Machine<State2> {}
}
impl Machine<State2> {
  fn do2(self) {}
}

Existential example:

enum Foo {}
unsafe extern "C" {
  fn create_foo() -> *mut Foo;
  fn read_foo(x: *const Foo) -> u32;
  fn write_foo(x: *mut Foo, y: u32);
}

In both cases the enum is not ever used as a generic argument in an enum like Option<!> or Result<u32, Infallible>. A good way to filter this out would be to check if the type is ever returned from a function as a generic parameter to a type which holds that type by-value only on some, potentially nested, variants.

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