Rollup merge of #138259 - compiler-errors:disentangle-ribs, r=BoxyUwU

Disentangle `ForwardGenericParamBan` and `ConstParamTy` ribs

In #137617, the `ConstParamTy` rib was adjusted to act kinda like the `ForwardGenericParamBan`. However, this means that it no longer served its purpose banning generics from *parent items*. Although we still are checking for param type validity using the `ConstParamTy_` trait, which means that we weren't accepting code we shouldn't, I think it's a bit strange for us not to be rejecting code like this during *resolution* and instead letting these malformed const generics leak into the type system:

```rust
trait Foo<T> {
  fn bar<const N: T>() {}
}
```

This PR does a few things:
1. Introduce a `ForwardGenericParamBanReason` enum, and start using the `ForwardGenericParamBan` rib to ban forward-declared params in const tys when `generic_const_parameter_types` is enabled.
2. Start using the `ConstParamTy` rib to ban *all* generics when `generic_const_parameter_types` is disabled.
3. Improve the diagnostics for both of the cases above, and for forward-declared params in parameter defaults too :3

r? `@BoxyUwU` or reassign

Author: Manishearth