Issue Description:

If you nest a single-case enum in another enum and try to reflect on the outer enum case, you cannot reflect on this inner value. For example, the following prints an empty array:

enum Inner { case innermost }
enum Outer { case inner(Inner) }
print(Array(Mirror(reflecting: Outer.inner(.innermost)).children))
// prints "[]"

On the other hand, adding a case to the single-case enum restores the ability to reflect on it:

enum Inner { case innermost, innermost2 }
enum Outer { case inner(Inner) }
print(Array(Mirror(reflecting: Outer.inner(.innermost)).children))
// prints "[(label: Optional("inner"), value: Inner.innermost)]"

My current workaround involves checking the memory layout size of Inner and if it's zero then I unsafely bitcast a value of Void.