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>

src/ast/binary_expression.h

diff --git a/src/ast/binary_expression.h b/src/ast/binary_expression.h
index 4ef24d8..e96d8da 100644
--- a/src/ast/binary_expression.h
+++ b/src/ast/binary_expression.h
@@ -48,7 +48,7 @@
 };
 
 /// An binary expression
-class BinaryExpression : public Expression {
+class BinaryExpression : public Castable<BinaryExpression, Expression> {
  public:
   /// Constructor
   BinaryExpression();