prefer_void_to_null should not be reported in a Cast Pattern or an As Expression

[srawlins]

void is not allowed in either:

void f(int a) {
  switch (a) {
    case var _ as void: // compile-time error
  }
  a as void; // compile-time error
}

So prefer_void_to_null should not be reported in those positions.

[pq]

Nice one!