cmd/protoc-gen-go: remove generation of XXX_OneofWrappers

Associate the oneof wrapper types with a message by conveying that
information to the associated MessageInfo.

Change-Id: Iabfca593850e1d6a89498a37eacbf22dbb73bd20
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185239
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/testprotos/irregular/test.pb.go b/internal/testprotos/irregular/test.pb.go
index 93092dc..c8ef2c4 100644
--- a/internal/testprotos/irregular/test.pb.go
+++ b/internal/testprotos/irregular/test.pb.go
@@ -95,13 +95,6 @@
 	return nil
 }
 
-// XXX_OneofWrappers is for the internal use of the proto package.
-func (*Message) XXX_OneofWrappers() []interface{} {
-	return []interface{}{
-		(*Message_OneofMessage)(nil),
-	}
-}
-
 type isMessage_Union interface {
 	isMessage_Union()
 }
@@ -198,6 +191,9 @@
 		return
 	}
 	file_irregular_irregular_proto_init()
+	file_irregular_test_proto_msgTypes[0].OneofWrappers = []interface{}{
+		(*Message_OneofMessage)(nil),
+	}
 	out := protoimpl.TypeBuilder{
 		File: protoimpl.DescBuilder{
 			RawDescriptor: file_irregular_test_proto_rawDesc,