cmd/protoc-gen-go: generate descriptor proto using v2 textpb

Changes:
* Modify protoc-gen-go to use a helper from internal/impl
to print the message as text.
* Add a helper function to internal/impl that calls v2 textpb.
* Modify encoding/textpb to avoid depending on descriptor proto,
which would cause an import cycle.
* Modify internal/fileinit to populate a pseudo-internal
method on MessageDescriptor to check whether the message should
use the message set wire format. We avoid adding this to the
main MessageDescriptor interface since message sets are a
deprecated proto1 feature that we should avoid promoting.

Change-Id: Ibaf79a563af695756f11ddc4db69b38e25a8f1a7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168439
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/fileinit/desc.go b/internal/fileinit/desc.go
index dbfe377..6d3b1b4 100644
--- a/internal/fileinit/desc.go
+++ b/internal/fileinit/desc.go
@@ -292,6 +292,7 @@
 		new func() pref.Message
 
 		isMapEntry      bool
+		isMessageSet    bool
 		fields          fieldDescs
 		oneofs          oneofDescs
 		resvNames       names
@@ -352,6 +353,12 @@
 	return md.lazy
 }
 
+// IsMessageSet is a pseudo-internal API for checking whether a message
+// should serialize in the proto1 message format.
+func (md *messageDesc) IsMessageSet() bool {
+	return md.lazyInit().isMessageSet
+}
+
 func (fd *fieldDesc) Options() pref.OptionsMessage {
 	return unmarshalOptions(ptype.X.FieldOptions(), fd.options)
 }
diff --git a/internal/fileinit/desc_lazy.go b/internal/fileinit/desc_lazy.go
index 0ff208a..ffcad72 100644
--- a/internal/fileinit/desc_lazy.go
+++ b/internal/fileinit/desc_lazy.go
@@ -560,6 +560,8 @@
 			switch num {
 			case descfield.MessageOptions_MapEntry:
 				md.lazy.isMapEntry = wire.DecodeBool(v)
+			case descfield.MessageOptions_MessageSetWireFormat:
+				md.lazy.isMessageSet = wire.DecodeBool(v)
 			}
 		default:
 			m := wire.ConsumeFieldValue(num, typ, b)