Const generic parameters are allowed in the wrong position (causing ICEs).

Note the `Array<N, ()>` use, instead of `Array<(), N>`:
```rust
#![feature(const_generics)]

pub type Array<T, const N: usize> = [T; N];

pub fn foo<const N: usize>() -> Array<N, ()> {
    unimplemented!()
}
```
This currently results in `cat_expr Errd` and `broken MIR` ICEs.

cc @varkor @yodaldevoid @oli-obk