rustdoc: Associated type fixes
The first commit fixes a bug with "dud" items in the search index from
misrepresented `type` items in trait impl blocks.
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.
The second commit fixes a bug that made signatures and where bounds
using associated types (if they were not on `Self`) incorrect.
The third commit fixes so that where clauses in type alias definititons
are shown.
Fixes#22442Fixes#24417Fixes#25769
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