rustdoc: emit JS paths for struct-like variants

rustdoc: emit JS paths for struct-like variants

On the backend, rustdoc now emits `paths` entries to a crate's search
index for struct-like enum variants, and index items of type structfield
which belong to such variants point to their variant parents in the
`paths` table, rather than their enum grandparents.  The path entry for
a variant is the fully qualified module path plus the enum name.

On the frontend, the search code recognizes structfields belonging to
structlike variants in the `paths` table and re-constructs the URL to
the field's anchor on the enum documentation page.

closes #16017