rustdoc fails to linkify paths containing turbofish on source code pages under `--generate-link-to-definition`

[fmease]

This is a regression that's caused by PR #135771.

Consider the following program:

fn scope() {
    let _: Option<i32>;   // OK    `Option` is linkified.
    let _: Option::<i32>; // FAIL  `Option` is *not* linkified.

    let _ = std::convert::identity(0);      // OK    `identity` is linkified
    let _ = std::convert::identity::<_>(0); // FAIL  `identity` is *not* linkified.

    let _: fn(()) = drop;     // OK    `drop` is linkified
    let _: fn(()) = drop::<>; // FAIL  `drop` is *not* linkified
}