core: Destroy data resources with "terraform destroy"

Previously they would get left behind in the state because we had no
support for planning their destruction. Now we'll create a "destroy" plan
and act on it by just producing an empty state on apply, thus ensuring
that the data resources don't get left behind in the state after
everything else is gone.

terraform/eval_read_data.go

@@ -80,6 +80,16 @@ func (n *EvalReadDataApply) Eval(ctx EvalContext) (interface{}, error) {
 	provider := *n.Provider
 	diff := *n.Diff
 
+	// If the diff is for *destroying* this resource then we'll
+	// just drop its state and move on, since data resources don't
+	// support an actual "destroy" action.
+	if diff.Destroy {
+		if n.Output != nil {
+			*n.Output = nil
+		}
+		return nil, nil
+	}
+
 	// For the purpose of external hooks we present a data apply as a
 	// "Refresh" rather than an "Apply" because creating a data source
 	// is presented to users/callers as a "read" operation.

terraform/transform_resource.go

@@ -735,6 +735,34 @@ func (n *graphNodeExpandedResource) dataResourceEvalNodes(resource *Resource, in
 		},
 	})
 
+	// Diff the resource for destruction
+	nodes = append(nodes, &EvalOpFilter{
+		Ops: []walkOperation{walkPlanDestroy},
+		Node: &EvalSequence{
+			Nodes: []EvalNode{
+
+				&EvalReadState{
+					Name:   n.stateId(),
+					Output: &state,
+				},
+
+				// Since EvalDiffDestroy doesn't interact with the
+				// provider at all, we can safely share the same
+				// implementation for data vs. managed resources.
+				&EvalDiffDestroy{
+					Info:   info,
+					State:  &state,
+					Output: &diff,
+				},
+
+				&EvalWriteDiff{
+					Name: n.stateId(),
+					Diff: &diff,
+				},
+			},
+		},
+	})
+
 	// Apply
 	nodes = append(nodes, &EvalOpFilter{
 		Ops: []walkOperation{walkApply, walkDestroy},