internal/impl: unexport Unwrapper

This interface is only ever used within the impl package.
Unexport it to avoid potential abuse by users.

Change-Id: I58d0cc2e8cb01c8c9d36cdec469c8d2196c4f836
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193197
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect_gen.go b/internal/impl/message_reflect_gen.go
index 5376920..9968277 100644
--- a/internal/impl/message_reflect_gen.go
+++ b/internal/impl/message_reflect_gen.go
@@ -21,9 +21,9 @@
 	return m.messageInfo().New()
 }
 func (m *messageState) Interface() protoreflect.ProtoMessage {
-	return m.ProtoUnwrap().(protoreflect.ProtoMessage)
+	return m.protoUnwrap().(protoreflect.ProtoMessage)
 }
-func (m *messageState) ProtoUnwrap() interface{} {
+func (m *messageState) protoUnwrap() interface{} {
 	return m.pointer().AsIfaceOf(m.messageInfo().GoReflectType.Elem())
 }
 func (m *messageState) ProtoMethods() *protoiface.Methods {
@@ -128,12 +128,12 @@
 	return m.messageInfo().New()
 }
 func (m *messageReflectWrapper) Interface() protoreflect.ProtoMessage {
-	if m, ok := m.ProtoUnwrap().(protoreflect.ProtoMessage); ok {
+	if m, ok := m.protoUnwrap().(protoreflect.ProtoMessage); ok {
 		return m
 	}
 	return (*messageIfaceWrapper)(m)
 }
-func (m *messageReflectWrapper) ProtoUnwrap() interface{} {
+func (m *messageReflectWrapper) protoUnwrap() interface{} {
 	return m.pointer().AsIfaceOf(m.messageInfo().GoReflectType.Elem())
 }
 func (m *messageReflectWrapper) ProtoMethods() *protoiface.Methods {