Analyzer should warn when A<void> is assign to A<?>

[a14n]

On sdk>=2.0 there should be a hint/error for things like:

void main() {
  List<String> l1 = List<void>(); // should be an error because it's an error at runtime
  List<String> l2 = <void>[]; // strangely there's already an error today
  List<dynamic> l3 = List<void>(); // OK for dynamic
}

[matanlurey]

These are downcasts, and are legal in the language.

Otherwise, this is duplicate of #33372.

[MichaelRFairhurst]

Also will drop in to say that this was intended to be illegal by dart 2 with "voidness preservation" (basically a form of illegal implicit downcasts that was on by default but limited to this case). Unfortunately we didn't have time to land it, but it may get a lint or be in dart 3.