Resolving document names from Index is expensive. when querying
type/constructor declarations for completion searches, we can
avoid reading document names from Index as much as possible
to boost performance.

Signed-off-by: Jinbo Wang <jinbwan@microsoft.com>
Change-Id: Ib93efd067dacc400e92903d44f8103a81a0ea665
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/184535
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Gayan Perera <gayanper@gmail.com>