[Augmentations] No error in the analyzer when enum's implicit getter is augmented

[sgrekhov]

According to the feature specification

Enum values can only be augmented by enum values, and the implicit getter introduced by them is not augmentable.

But the analyzer doesn't produce the following expected errors.

enum E2 {
  e0;
}

enum E3 {
  e0;
}

augment enum E2 {
  e1;
  augment static E2 get e0 => e0;
//^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
}

augment enum E3 {
  e1;
  augment static E3 get e0 => augmented;
//^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
}