Rollup merge of #89234 - nbdd0121:discr, r=jackh726

Rollup merge of #89234 - nbdd0121:discr, r=jackh726

Disallow non-c-like but "fieldless" ADTs from being casted to integer if they use arbitrary enum discriminant

Code like

```rust
#[repr(u8)]
enum Enum {
    Foo /* = 0 */,
    Bar(),
    Baz{}
}

let x = Enum::Bar() as u8;
```

seems to be unintentionally allowed so we couldn't disallow them now ~~, but we could disallow them if arbitrary enum discriminant is used before 1.56 hits stable~~ (stabilization was reverted).

Related: #88621

`@rustbot` label +T-lang