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/encoding/bench_test.go b/encoding/bench_test.go
index 32c81d3..88f62b7 100644
--- a/encoding/bench_test.go
+++ b/encoding/bench_test.go
@@ -45,7 +45,7 @@
 	}
 
 	knownFields := m.KnownFields()
-	fieldDescs := m.Type().Fields()
+	fieldDescs := m.Descriptor().Fields()
 	for i := 0; i < fieldDescs.Len(); i++ {
 		fd := fieldDescs.Get(i)
 		num := fd.Number()