Issue Description:

The Swift compiler emits a warning when a variable is inferred to have a type that is an empty enum like Never. An easy way to trigger this is by assigning the result of a function that returns Never to a variable:

let never = fatalError() // warning: constant 'never' inferred to have type 'Never', which is an enum with no cases

However, the compiler also emits this warning for variables inferred to be Never?, but warning is not accurate in that case, because Never? is an enum with two cases, `some` and `none` (though of course, a valid of Never?.some is as impossible as a value of Never).

func makeOptionalNever() -> Never? {
    return nil
}

let optionalNever = makeOptionalNever() // warning: constant 'optionalNever' inferred to have type 'Never?', which is an enum with no cases

The compiler does not emit the warning for variables inferred to be Never??:

func makeDoubleOptionalNever() -> Never?? {
    return nil
}

let doubleOptionalNever = makeDoubleOptionalNever() // no warning