rustdoc: search links to non-existent tuple field anchors

[jsha]

Steps to reproduce:

1. Search for 0: https://doc.rust-lang.org/nightly/std/index.html?search=0
2. Click on first result (std::borrow::Cow::Borrowed::0

Expected result:

Either (a) Borrowed::0 isn't listed in the search index since it's not a named type, or (b) the hyperlink goes to #variant.Borrowed (which exists) instead of #variant.Borrowed.field.0 (which doesn't exist).

[notriddle]

Fixed by #103799