castable.h: Fix warning

> clang --version
Apple clang version 11.0.3 (clang-1103.0.32.29)

../src/castable.h:199:11: error: definition of implicit copy assignment operator for 'CastableBase' is deprecated because it has a user-declared destructor [-Werror,-Wdeprecated]

Seems to only be raised with this particular flavor of clang?

Change-Id: I9a549c753cd28a11855138a97823e7c1c92fab25
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49532
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/castable.h

@@ -196,8 +196,14 @@ class CastableBase {
   /// Copy constructor
   CastableBase(const CastableBase&) = default;
 
+  /// Destructor
   virtual ~CastableBase() = default;
 
+  /// Copy assignment
+  /// @param other the CastableBase to copy
+  /// @returns the new CastableBase
+  CastableBase& operator=(const CastableBase& other) = default;
+
   /// @returns the TypeInfo of the object
   virtual const tint::TypeInfo& TypeInfo() const = 0;