Rollup merge of #157438 - qaijuang:rustdoc-hidden-assoc-type-link, r=notriddle

rustdoc: don't link doc(hidden) associated type projections

Rustdoc renders associated type projections like `T::Foo` by linking `Foo` to the containing trait page and appending `#associatedtype.Foo`.

For `#[doc(hidden)]` associated types, that anchor is not rendered in normal docs, so this can produce dangling links.

This PR checks the corresponding trait associated item before emitting the projection link.
When the associated type is hidden, rustdoc leaves the projection text plain, accounting for `--document-hidden-items` as well.

Fixes rust-lang/rust#151454.

Author: jhpratt