rustdoc: Skip types in impls in search index

rustdoc: Skip types in impls in search index

For a trait *implementation* there are typedefs which are the types for
that particular trait and implementor. Skip these in the search index.

There were lots of dud items in the search index due to this (search for
Item, Iterator's associated type).

Add a boolean to clean::TypedefItem so that it tracks whether the it is
a type alias on its own, or if it's a `type` item in a trait impl.

Fixes #22442