cmd/protoc-gen-go: generate message fields

This produces exactly the same output as github.com/golang/protobuf.

Change-Id: I01aacc9277c5cb5b4cc295f5ee8af12b4a524781
Reviewed-on: https://go-review.googlesource.com/134955
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/testdata/proto2/enum.pb.go b/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
index 85b2fad..e8e053c 100644
--- a/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
+++ b/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
@@ -165,6 +165,9 @@
 }
 
 type EnumContainerMessage1 struct {
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
 }
 
 // NestedEnumType2A comment.
@@ -244,6 +247,9 @@
 }
 
 type EnumContainerMessage1_EnumContainerMessage2 struct {
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
 }
 
 func init() {