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/jsonpb/decode.go b/encoding/jsonpb/decode.go
index 0a08e4f..7e142c5 100644
--- a/encoding/jsonpb/decode.go
+++ b/encoding/jsonpb/decode.go
@@ -135,7 +135,7 @@
 func (o UnmarshalOptions) unmarshalMessage(m pref.Message, skipTypeURL bool) error {
 	var nerr errors.NonFatal
 
-	if isCustomType(m.Type().FullName()) {
+	if isCustomType(m.Descriptor().FullName()) {
 		return o.unmarshalCustomType(m)
 	}
 
@@ -160,9 +160,9 @@
 	var seenNums set.Ints
 	var seenOneofs set.Ints
 
-	msgType := m.Type()
+	messageDesc := m.Descriptor()
 	knownFields := m.KnownFields()
-	fieldDescs := msgType.Fields()
+	fieldDescs := messageDesc.Fields()
 	xtTypes := knownFields.ExtensionTypes()
 
 Loop:
@@ -208,7 +208,9 @@
 					xtTypes.Register(xt)
 				}
 			}
-			fd = xt
+			if xt != nil {
+				fd = xt.Descriptor()
+			}
 		} else {
 			// The name can either be the JSON name or the proto field name.
 			fd = fieldDescs.ByJSONName(name)
@@ -225,13 +227,13 @@
 				}
 				continue
 			}
-			return newError("%v contains unknown field %s", msgType.FullName(), jval)
+			return newError("%v contains unknown field %s", messageDesc.FullName(), jval)
 		}
 
 		// Do not allow duplicate fields.
 		num := uint64(fd.Number())
 		if seenNums.Has(num) {
-			return newError("%v contains repeated field %s", msgType.FullName(), jval)
+			return newError("%v contains repeated field %s", messageDesc.FullName(), jval)
 		}
 		seenNums.Set(num)