Derive all ast::Node from Castable

The hand-rolled `AsBlah()`, `IsBlah()` methods will be migrated in future changes.

Change-Id: I078c100b561b50018771cc38c1cac4379c393424
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34301
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/src/ast/literal.h b/src/ast/literal.h
index 2154b2b..b8c1b7c 100644
--- a/src/ast/literal.h
+++ b/src/ast/literal.h
@@ -31,7 +31,7 @@
 class UintLiteral;
 
 /// Base class for a literal value
-class Literal : public Node {
+class Literal : public Castable<Literal, Node> {
  public:
   ~Literal() override;