CppEditor: "Include Hierarchy": using snapshot from snapshotUpdater

For "includes" we should use snapshot from "SnapshotUpdater"(project
specific snapshot, not global snapshot) instead of "global" snapshot.
The snapshot of the editor's SnapshotUpdater contains the documents
parsed with the appropriate include paths/defines etc. and should
therefore be used for the include hierarchy.

Change-Id: I6bbaf5040536b771d32697aad1db364758ff8382
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>