Shared discriminator does not cause compiler error

The following code should generate a compiler error stating how the same discriminant value is assigned multiple times.
```rust
enum Foo {
    Bar = 1,
    Qaz = 1,
}

fn main() {}
```
But it seems to generate no errors on gccrs.
Godbolt: https://godbolt.org/z/f1aY19v4q