Tracking issue for treating tuple struct and variant constructors as const fn

The `const_constructor` feature allows calling any expression of a tuple-like constructor in a `const fn`:

```rust
const fn make_options() {
    Some(0); // OK on stable
    (Option::Some)(1); // OK on stable
    let f = Option::Some;
    f(2); // Requires feature gate
}
```

cc @oli-obk @Centril 