`impl const` wrongly accepts impl with non-const provided methods

This prints no error, but `prov` is not `const`-safe, so the `impl const Tr` should not be accepted:

```rust
#![feature(const_trait_impl)]

trait Tr {
    fn req(&self);
    
    fn prov(&self) {
        println!("lul");
        self.req();
    }
}

struct S;

impl const Tr for S {
    fn req(&self) {}
}
```