Disallow Array[Nothing], Array[Null], ... to avoid ClassCastException in some cases

See also https://issues.scala-lang.org/browse/SI-7453

In scalac:

scala> if (false) Array("qwe") else Array()
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

In dotty, thankfully:

scala> if (false) Array("qwe") else Array() 
res1: Array[String] | Array[Nothing] = [Ljava.lang.Object;@1dd92fe2

But still in dotty:

scala> val x = if (false) Array("qwe") else Array() 
x: Array[String] | Array[Nothing] = [Ljava.lang.Object;@5454d35e
scala> val y: Array[_ <: String] = x 
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

We either need to disallow Array[Nothing], Array[Null], etc or we need to erase Array[_ <: X] to Object instead of [X;.

It would also be much nicer if we could infer the empty array above to have type Array[String] instead of Array[Nothing], I don’t know what prevents us from doing this currently.

[odersky]

One quite radical idea would be to treat Nothing and Null not as classes but as abstract types. That means we could not generate a ClassTag for them and therefore it would be impossible to create instances of Array[Nothing] and Array[Null]`

[odersky]

The comment says that we might want to consider disallowing forming a ClassTag[Null] or ClassTag[Nothing] manually (they are not synthesized anymore). On the other hand, anyone who would do that in a sense does homegrown meta-programming, and it’s not clear to what extent we want to give soundness guarantees for such code. So I think it’s OK to close for now. If we feel like it, revisit the manual creation issue.