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/benchmarks/benchmarks.pb.go b/internal/testprotos/benchmarks/benchmarks.pb.go
index c07f40a..7490e11 100644
--- a/internal/testprotos/benchmarks/benchmarks.pb.go
+++ b/internal/testprotos/benchmarks/benchmarks.pb.go
@@ -157,11 +157,11 @@
 	(*BenchmarkDataset)(nil), // 0: benchmarks.BenchmarkDataset
 }
 var file_benchmarks_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_benchmarks_proto_init() }