all: remove use of deprecated NewMessage

Replace NewMessage calls with NewField, NewElement, or NewValue.

Change-Id: I6d2bb4f11f0eb2ba7a52308b1addb111137ad4b9
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193266
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/proto/decode_gen.go b/proto/decode_gen.go
index ce53dcd..61d5f8a 100644
--- a/proto/decode_gen.go
+++ b/proto/decode_gen.go
@@ -574,11 +574,11 @@
 		if n < 0 {
 			return 0, wire.ParseError(n)
 		}
-		m := list.NewMessage()
-		if err := o.unmarshalMessage(v, m); err != nil {
+		m := list.NewElement()
+		if err := o.unmarshalMessage(v, m.Message()); err != nil {
 			return 0, err
 		}
-		list.Append(protoreflect.ValueOf(m))
+		list.Append(m)
 		return n, nil
 	case protoreflect.GroupKind:
 		if wtyp != wire.StartGroupType {
@@ -588,11 +588,11 @@
 		if n < 0 {
 			return 0, wire.ParseError(n)
 		}
-		m := list.NewMessage()
-		if err := o.unmarshalMessage(v, m); err != nil {
+		m := list.NewElement()
+		if err := o.unmarshalMessage(v, m.Message()); err != nil {
 			return 0, err
 		}
-		list.Append(protoreflect.ValueOf(m))
+		list.Append(m)
 		return n, nil
 	default:
 		return 0, errUnknown