rustdoc: Always warn when linking from public to private items

rustdoc: Always warn when linking from public to private items

Change the logic such that linking from a public to a private item always
triggers intra_doc_link_resolution_failure. Previously, the warning was
not emitted when --document-private-items is passed.

Also don't rely anymore on the item's visibility, which would falsely trigger
the lint now that the check for --document-private-items is gone.