reflect/protoreflect: rename methods with Type suffix

The protobuf type system uses the word "descriptor" instead of "type".
We should avoid the "type" verbage when we aren't talking about Go types.
The old names are temporarily kept around for compatibility reasons.

Change-Id: Icc99c913528ead011f7a74aa8399d9c5ec6dc56e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172238
Reviewed-by: Herbie Ong <herbie@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/textpb/decode.go b/encoding/textpb/decode.go
index 218d95f..863911f 100644
--- a/encoding/textpb/decode.go
+++ b/encoding/textpb/decode.go
@@ -164,7 +164,7 @@
 			}
 		} else {
 			// If field is a oneof, check if it has already been set.
-			if od := fd.OneofType(); od != nil {
+			if od := fd.Oneof(); od != nil {
 				idx := uint64(od.Index())
 				if seenOneofs.Has(idx) {
 					return errors.New("oneof %v is already set", od.FullName())
@@ -313,7 +313,7 @@
 		}
 		if name, ok := input.Name(); ok {
 			// Lookup EnumNumber based on name.
-			if enumVal := fd.EnumType().Values().ByName(name); enumVal != nil {
+			if enumVal := fd.Enum().Values().ByName(name); enumVal != nil {
 				return pref.ValueOf(enumVal.Number()), nil
 			}
 		}
@@ -356,7 +356,7 @@
 // unmarshalMap unmarshals given []text.Value into given protoreflect.Map.
 func (o UnmarshalOptions) unmarshalMap(input []text.Value, fd pref.FieldDescriptor, mmap pref.Map) error {
 	var nerr errors.NonFatal
-	fields := fd.MessageType().Fields()
+	fields := fd.Message().Fields()
 	keyDesc := fields.ByNumber(1)
 	valDesc := fields.ByNumber(2)