`const fn` allows unreachable calls to non-`const` fns

E.g. the following compiles on stable:
```rust
const fn f() {
    return;
    fn f() {}
    f();
}
```

@eddyb notes that this is because we do a "semantic check" and that `f();` is not linked in the CFG.

However, it seems to me that the basic blocks should be there (tho I haven't checked yet...) so you can visit the call to `f();` and ban it.

cc @oli-obk 