[cfe] Private field of a mixin application is not promoted

[sgrekhov]

The following test works in analyzer and fails in CFE

mixin Mixin {
  final int? _x = 42;

  void test() {
    if (_x != null) {
      _x.isOdd; // Ok
    }
  }
}

base mixin BaseMixin {
  final int? _x = 42;

  void test() {
    if (_x != null) {
      _x.isOdd; // Ok
    }
  }
}

class WithMixin = Object with Mixin;

base class WithBaseMixin = Object with BaseMixin;

main() {
  WithMixin _withMixin = WithMixin();
  if (_withMixin._x != null) {
    _withMixin._x.isEven; // CFE. Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
  }
  _withMixin.test();

  WithBaseMixin _withBaseMixin = WithBaseMixin();
  if (_withBaseMixin._x != null) {
    _withBaseMixin._x.isEven; // CFE. Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
  }
  _withBaseMixin.test();
}

cc @stereotype441

Tested on the edge SDK (Sept 6, 2023) on Linux x64