Issue Description:

The following example prints `false`, because the `case nil, nil` is not matched by the passed value. This is because `case nil, nil` is treated as two separate patterns, equivalent to:

case nil:
    fallthrough
case nil:
    ...

func check(_ arg1: Int?, _ arg2: Int?) -> Bool {
    switch (arg1, arg2) {
    case (.some(let val1), .some(let val2)):
        return val1 > val2
    case nil, nil:
        return true
    default:
        return false
    }
}

print(check(nil, nil))

We have a check that diagnoses and warns about other duplicate literals, so we should probably extend that to `nil` literals as well!