Imprecise printing of extension methods for -Xprint:front

## minimized code

```Scala
def (impl: Impl) prop given Int = ???
```

compiled with `-Xprint:front` prints:

```Scala
(impl: Impl) def prop given (x$1: Int): Nothing = ???
```

## expectation

```Scala
def (impl: Impl) prop given (x$1: Int): Nothing = ???
```

