[CFE] Type `void` and null-aware selector issue

[sgrekhov]

The code below produces no issues in the analyzer but an error in CFE.

class C<T> {
  T t;
  C(this.t);
}

main() {
  C<void> c1 = C<void>(1);
  C<void>? c2 = (2 > 1) ? C<void>(2) : null;
  print(c1.t as int); // prints 1
  print(c2?.t as int); // Error: This expression has type 'void' and can't be used.
}

Accoding to the spec

In a type cast e as T, e may have type void

There should be no such error in the CFE.

`Dart SDK version: 3.7.0-edge.a0c4efddb8ada994c3f8568268478a4948bc32b7 (main) (Tue Nov 12 08:16:42 2024 +0000) on "linux_x64"

[lrhn]

Likely a result of eagerly desugaring c2?.t to let $tmp = c2 in $tmp == null ? $tmp.t : null where $tmp.t is now an expression of type void inside a conditional expression which then also has type void, inside a let expression which is not a position on the allow list ... because let expressions don't exist in the language.

The allow-list, and errors in general, should be applied only to the user's source, not our own desugaring.
The user source contains no expression with type void which is not in an allowed position.