A "trivial" enum is one that carries no information.
The previous logic asserted that a trivial enum had only one
case and that case had no payload.
But this is also a trivial payload:
```
// Trivial, so zero-sized
enum MyFormatVersion {
case v4
}
// Also trivial, since the payload carries no information
enum Format {
case MyFormat(MyFormatVersion)
}
```
This commit adds a test case similar to the above
and corrects the assertions for trivial enums to
assert that there is either no payload, or that the
payload is zero-sized.
Resolves rdar://116406504
