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/encoding/protojson/decode.go b/encoding/protojson/decode.go
index 5e32d8b..3e1deed 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -183,7 +183,7 @@
 				return errors.New("unable to resolve [%v]: %v", extName, err)
 			}
 			if extType != nil {
-				fd = extType.Descriptor()
+				fd = extType.TypeDescriptor()
 			}
 		} else {
 			// The name can either be the JSON name or the proto field name.
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index 7207dde..79e1eaf 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -127,7 +127,7 @@
 				return errors.New("unable to resolve [%v]: %v", extName, err)
 			}
 			if xt != nil {
-				fd = xt.Descriptor()
+				fd = xt.TypeDescriptor()
 			}
 		}