all: rename ExtensionType Descriptor method to TypeDescriptor (1/2)

Descriptor methods generally return a Descriptor with no Go type
information. ExtensionType's Descriptor is an exception, returning an
ExtensionTypeDescriptor containing both the proto descriptor and a
reference back to the ExtensionType. The pure descriptor is accessed
by xt.Descriptor().Descriptor().

Rename ExtensionType's Descriptor method to TypeDescriptor to make it
clear that it behaves a bit differently.

Change 1/2: Add the TypeDescriptor method and deprecate Descriptor.

Change-Id: I1806095044d35a474d60f94d2a28bdf528f12238
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/192139
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/proto/extension.go b/proto/extension.go
index 2e1c78f..809c12d 100644
--- a/proto/extension.go
+++ b/proto/extension.go
@@ -10,13 +10,13 @@
 
 // HasExtension reports whether an extension field is populated.
 func HasExtension(m Message, ext protoreflect.ExtensionType) bool {
-	return m.ProtoReflect().Has(ext.Descriptor())
+	return m.ProtoReflect().Has(ext.TypeDescriptor())
 }
 
 // ClearExtension clears an extension field such that subsequent
 // HasExtension calls return false.
 func ClearExtension(m Message, ext protoreflect.ExtensionType) {
-	m.ProtoReflect().Clear(ext.Descriptor())
+	m.ProtoReflect().Clear(ext.TypeDescriptor())
 }
 
 // GetExtension retrieves the value for an extension field.
@@ -24,10 +24,10 @@
 // If the field is unpopulated, it returns the default value for
 // scalars and an immutable, empty value for lists, maps, or messages.
 func GetExtension(m Message, ext protoreflect.ExtensionType) interface{} {
-	return ext.InterfaceOf(m.ProtoReflect().Get(ext.Descriptor()))
+	return ext.InterfaceOf(m.ProtoReflect().Get(ext.TypeDescriptor()))
 }
 
 // SetExtension stores the value of an extension field.
 func SetExtension(m Message, ext protoreflect.ExtensionType, value interface{}) {
-	m.ProtoReflect().Set(ext.Descriptor(), ext.ValueOf(value))
+	m.ProtoReflect().Set(ext.TypeDescriptor(), ext.ValueOf(value))
 }