In particular, the exactly internal layout of Array<> is quite
different between macOS and Linux, which makes it impractical
for use in tests like this. An empty class is still a reference
and doesn't introduce so many complications.
A "generic" case is any case whose payload type depends
on generic type parameters for the enum or any enclosing type.
Some examples:
```
struct S<T> {
enum E {
case a // Non-generic case
case b(Int) // Non-generic case
case c(T) // Generic case
case d([U]) // Generic case
case e([Int]) // Non-generic case
}
}
```
This is important for correctly distinguishing MPE versus
SPE layouts. A case is considered "empty" only if it
is either a non-payload case (`case a`) or if the payload
is zero-sized and non-generic. Generic cases are always
treated as non-zero-sized for purposes of determining
the layout strategy.
Correctly testing this is tricky, since some of the
layout strategies differ only in whether they export
extra tag values as XIs. The easiest way to verify is
to check whether wrapping the result in an `Optional<>`
adds another byte to the overall size.
