Within a larger expression, when the type of `Box::new(T::default())` is
`Box<dyn Trait>`, the concrete type `T` cannot be omitted in the
proposed replacement `Box::<T>::default()`.
fix `box-default` ignoring trait objects' types
This avoids removing the turbofish when the `Box` type is a `dyn` or `impl _`.
This fixes#9621.
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changelog: none
