Use PDG to remove NON_NULL from pointers in static analysis

Remove the NON_NULL permission from all nodes in
the null graph from the PDG.

c2rust-analyze/src/analyze.rs

@@ -993,7 +993,10 @@ fn run(tcx: TyCtxt) {
                 if !node_info.unique {
                     perms.remove(PermissionSet::UNIQUE);
                 }
-                // TODO: PermissionSet::NON_NULL
+                if let Some(true) = g.is_null {
+                    // TODO: is this enough?
+                    perms.remove(PermissionSet::NON_NULL);
+                }
 
                 if perms != old_perms {
                     let added = perms & !old_perms;