Auto merge of #109876 - jsha:uniquify-intra-doc, r=notriddle

rustdoc: make intra-doc link pass non-quadratic for repeated links

In the collect_intra_doc_links pass, links to a given item that occurred repeatedly were getting inserted into a `Vec<clean::ItemLink>` repeatedly. This led to n^2 behavior (where n = the number of pages generated), particularly for the intra-doc link on the `Into<U> for T where U: From<T>` blanket implementation, since that link appears on every single struct page.

Fixes #109851

Author: bors