rustdoc-search: handle `impl Into<X>` better
This PR fixes two bugs I ran into while searching the compiler docs:
- It omitted an `impl Trait` entry in the type signature field, producing `TyCtxt, , Symbol -> bool`
- It didn't let me search for `TyCtxt, DefId, Symbol -> bool` even though that's a perfectly good description of the function I was looking for (the function actually used `impl Into<DefId>`
r? ``@GuillaumeGomez`` cc ``@lolbinarycat``
d9ba4bf6fe