rustdoc creates links to doc(hidden) associated type

[RalfJung]

This came up in #151365 if one takes just the first commit: the link checker started complaining about some rustdoc-generated files.

	core/marker/trait.UnsafeUnpin.html:124: broken link fragment `#associatedtype.NonZeroInner` pointing to `core/num/trait.ZeroablePrimitive.html`
	core/num/struct.NonZero.html:3880: broken link fragment `#associatedtype.NonZeroInner` pointing to `core/num/trait.ZeroablePrimitive.html`
	std/marker/trait.UnsafeUnpin.html:195: broken link fragment `#associatedtype.NonZeroInner` pointing to `std/num/trait.ZeroablePrimitive.html`
	std/num/struct.NonZero.html:4108: broken link fragment `#associatedtype.NonZeroInner` pointing to `std/num/trait.ZeroablePrimitive.html`

NonZeroInner is a doc(hidden) associated type of the ZeroablePrimitive trait. It seems like the changes in this commit cause rustdoc to render trait instances that refer to this associated type, with links to the type definition, but since the type is hidden those links are dangling.

[qaijuang]

I was able to reproduce with a small crate containing:

pub trait ZeroablePrimitive {
    #[doc(hidden)]
    type NonZeroInner;
}

pub struct NonZero<T: ZeroablePrimitive>(pub T::NonZeroInner);

It generated links to trait.ZeroablePrimitive.html#associatedtype.NonZeroInner, but the trait page has no id="associatedtype.NonZeroInner"

[qaijuang]

@rustbot claim