Add move constructor to CastableBase.

This CL adds the Move constructor to castable base as the default one is
deleted due to the defined destructor and the Node subclass defines a
Move constructor.

Change-Id: I0eaac140719e74adfab1aeccf6ea663faff031e5
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34580
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/castable.h

@@ -48,6 +48,9 @@ class ClassID {
 /// @see Castable
 class CastableBase {
  public:
+  /// Move constructor
+  CastableBase(CastableBase&&) = default;
+
   virtual ~CastableBase() = default;
 
   /// @returns true if this object is of, or derives from a class with the