core: EvalDiffDestroy only update state if requested

The earlier change 5f07201a made it so that the state is always rewritten
by EvalDiffDestroy, but that was too disruptive to other users of
EvalDiffDestroy.

Now we follow the lead of EvalDiff and have a separate pointer for the
_output_ state, which allows the caller to opt in to having its state
pointer updated to reflect the new (nil) state.

NodePlannableResourceInstanceOrphan is the only caller that currently opts
in to this, since that was the focus of 5f07201a. We may need to make a
similar change to other plannable resource destroy nodes, but we'll wait
to see if that needs to be done in a subsequent commit.

terraform/eval_diff.go

@@ -427,6 +427,7 @@ type EvalDiffDestroy struct {
 	Addr        addrs.ResourceInstance
 	State       **InstanceState
 	Output      **InstanceDiff
+	OutputState **InstanceState
 }
 
 // TODO: test
@@ -463,9 +464,9 @@ func (n *EvalDiffDestroy) Eval(ctx EvalContext) (interface{}, error) {
 	// Update our output
 	*n.Output = diff
 
-	if n.State != nil {
+	if n.OutputState != nil {
 		// Record our proposed new state, which is nil because we're destroying.
-		*n.State = nil
+		*n.OutputState = nil
 	}
 
 	return nil, nil

terraform/node_resource_plan_orphan.go

@@ -46,8 +46,9 @@ func (n *NodePlannableResourceInstanceOrphan) EvalTree() EvalNode {
 			},
 			&EvalDiffDestroy{
 				Addr:        addr.Resource,
-				State:  &state, // Will point to a nil state after this complete, signalling destroyed
+				State:       &state,
 				Output:      &diff,
+				OutputState: &state, // Will point to a nil state after this complete, signalling destroyed
 			},
 			&EvalCheckPreventDestroy{
 				Addr:   addr.Resource,