cmd/protoc-gen-go: used named types for special fields

Using a named fields gives us the flexibility to change the underlying
representation of special fields without needing to regenerate user code.

We add a named type for ExtensionFields, UnknownFields, and SizeCache.

Change-Id: I107cf82899850ea76665310ce79def60f0f7ab97
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172402
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/cmd/protoc-gen-go/testdata/proto2/enum.pb.go b/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
index 071d96a..d73fa9e 100644
--- a/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
+++ b/cmd/protoc-gen-go/testdata/proto2/enum.pb.go
@@ -321,11 +321,11 @@
 }
 
 type EnumContainerMessage1 struct {
-	DefaultDuplicate1    *EnumType2 `protobuf:"varint,1,opt,name=default_duplicate1,json=defaultDuplicate1,enum=goproto.protoc.proto2.EnumType2,def=1" json:"default_duplicate1,omitempty"`
-	DefaultDuplicate2    *EnumType2 `protobuf:"varint,2,opt,name=default_duplicate2,json=defaultDuplicate2,enum=goproto.protoc.proto2.EnumType2,def=1" json:"default_duplicate2,omitempty"`
-	XXX_NoUnkeyedLiteral struct{}   `json:"-"`
-	XXX_unrecognized     []byte     `json:"-"`
-	XXX_sizecache        int32      `json:"-"`
+	DefaultDuplicate1    *EnumType2              `protobuf:"varint,1,opt,name=default_duplicate1,json=defaultDuplicate1,enum=goproto.protoc.proto2.EnumType2,def=1" json:"default_duplicate1,omitempty"`
+	DefaultDuplicate2    *EnumType2              `protobuf:"varint,2,opt,name=default_duplicate2,json=defaultDuplicate2,enum=goproto.protoc.proto2.EnumType2,def=1" json:"default_duplicate2,omitempty"`
+	XXX_NoUnkeyedLiteral struct{}                `json:"-"`
+	XXX_unrecognized     protoimpl.UnknownFields `json:"-"`
+	XXX_sizecache        protoimpl.SizeCache     `json:"-"`
 }
 
 func (x *EnumContainerMessage1) Reset() {
@@ -369,9 +369,9 @@
 }
 
 type EnumContainerMessage1_EnumContainerMessage2 struct {
-	XXX_NoUnkeyedLiteral struct{} `json:"-"`
-	XXX_unrecognized     []byte   `json:"-"`
-	XXX_sizecache        int32    `json:"-"`
+	XXX_NoUnkeyedLiteral struct{}                `json:"-"`
+	XXX_unrecognized     protoimpl.UnknownFields `json:"-"`
+	XXX_sizecache        protoimpl.SizeCache     `json:"-"`
 }
 
 func (x *EnumContainerMessage1_EnumContainerMessage2) Reset() {