core: EvalReadDataDiff to output direct config value alongside planned

We're now writing the "planned new value" to OutputValue, but the data
resource nodes during refresh need to see the verbatim config value in
order to decide whether read must be deferred to the apply phase, so we'll
optionally export that here too.

terraform/eval_read_data.go

@@ -24,6 +24,7 @@ type EvalReadDataDiff struct {
 
 	Output            **plans.ResourceInstanceChange
 	OutputValue       *cty.Value
+	OutputConfigValue *cty.Value
 	OutputState       **states.ResourceInstanceObject
 
 	// Set Previous when re-evaluating diff during apply, to ensure that
@@ -114,10 +115,12 @@ func (n *EvalReadDataDiff) Eval(ctx EvalContext) (interface{}, error) {
 	if n.Output != nil {
 		*n.Output = change
 	}
-
 	if n.OutputValue != nil {
 		*n.OutputValue = change.After
 	}
+	if n.OutputConfigValue != nil {
+		*n.OutputConfigValue = configVal
+	}
 
 	if n.OutputState != nil {
 		state := &states.ResourceInstanceObject{

terraform/node_data_refresh.go

@@ -150,7 +150,7 @@ func (n *NodeRefreshableDataResourceInstance) EvalTree() EvalNode {
 				ProviderAddr:      n.ResolvedProvider,
 				ProviderSchema:    &providerSchema,
 				Output:            &change,
-				OutputValue:    &configVal,
+				OutputConfigValue: &configVal,
 				OutputState:       &state,
 			},