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

@@ -22,9 +22,10 @@ type EvalReadDataDiff struct {
 	ProviderAddr   addrs.AbsProviderConfig
 	ProviderSchema **ProviderSchema
 
-	Output      **plans.ResourceInstanceChange
-	OutputValue *cty.Value
-	OutputState **states.ResourceInstanceObject
+	Output            **plans.ResourceInstanceChange
+	OutputValue       *cty.Value
+	OutputConfigValue *cty.Value
+	OutputState       **states.ResourceInstanceObject
 
 	// Set Previous when re-evaluating diff during apply, to ensure that
 	// the "Destroy" flag is preserved.
@@ -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

@@ -145,13 +145,13 @@ func (n *NodeRefreshableDataResourceInstance) EvalTree() EvalNode {
 			},
 
 			&EvalReadDataDiff{
-				Addr:           addr.Resource,
-				Config:         n.Config,
-				ProviderAddr:   n.ResolvedProvider,
-				ProviderSchema: &providerSchema,
-				Output:         &change,
-				OutputValue:    &configVal,
-				OutputState:    &state,
+				Addr:              addr.Resource,
+				Config:            n.Config,
+				ProviderAddr:      n.ResolvedProvider,
+				ProviderSchema:    &providerSchema,
+				Output:            &change,
+				OutputConfigValue: &configVal,
+				OutputState:       &state,
 			},
 
 			// The rest of this pass can proceed only if there are no