cmd/protoc-gen-go: annotate depIdxs list with index comments

Generate the current index into depIdxs for easier human debugging.

Change-Id: Ida42aa95137b2044a4dc267c31cebec5023bdfb1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/190278
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/testprotos/test/test_public.pb.go b/internal/testprotos/test/test_public.pb.go
index f694f37..0055855 100644
--- a/internal/testprotos/test/test_public.pb.go
+++ b/internal/testprotos/test/test_public.pb.go
@@ -77,11 +77,11 @@
 	(*PublicImportMessage)(nil), // 0: goproto.proto.test.PublicImportMessage
 }
 var file_test_test_public_proto_depIdxs = []int32{
-	0, // starting offset of method output_type sub-list
-	0, // starting offset of method input_type sub-list
-	0, // starting offset of extension type_name sub-list
-	0, // starting offset of extension extendee sub-list
-	0, // starting offset of field type_name sub-list
+	0, // [0:0] is the sub-list for method output_type
+	0, // [0:0] is the sub-list for method input_type
+	0, // [0:0] is the sub-list for extension type_name
+	0, // [0:0] is the sub-list for extension extendee
+	0, // [0:0] is the sub-list for field type_name
 }
 
 func init() { file_test_test_public_proto_init() }