reflect/protoreflect: add Descriptor specific methods

Added methods:
	Enum.Descriptor
	Message.Descriptor
	EnumType.Descriptor
	MessageType.Descriptor
	ExtensionType.Descriptor
	Message.New

All functionality is switched over to use those methods instead of
implicitly relying on the fact that {Enum,Message}Type implicitly
implement the associated descriptor interface.

This CL does not yet remove {Enum,Message}.Type or prevent
{Enum,Message,Extension}Type from implementating a descriptor.
That is a subsequent CL.

The Message.New method is also added to replace functionality
that will be lost when the Type methods are removed.

Change-Id: I7fefde1673bbd40bfdac489aca05cec9a6c98eb1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/174918
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/legacy/enum.go b/internal/legacy/enum.go
index 0ee397f..2a57b0b 100644
--- a/internal/legacy/enum.go
+++ b/internal/legacy/enum.go
@@ -55,12 +55,16 @@
 	goTyp reflect.Type
 }
 
-func (e *enumWrapper) Number() pref.EnumNumber {
-	return e.num
-}
+// TODO: Remove this.
 func (e *enumWrapper) Type() pref.EnumType {
 	return e.pbTyp
 }
+func (e *enumWrapper) Descriptor() pref.EnumDescriptor {
+	return e.pbTyp.Descriptor()
+}
+func (e *enumWrapper) Number() pref.EnumNumber {
+	return e.num
+}
 func (e *enumWrapper) ProtoReflect() pref.Enum {
 	return e
 }