tint/sem: Add Declaration() override for IndexAccessorExpression

Convenience overload to return ast::IndexAccessorExpression instead of ast::Expression

Change-Id: Ib239fd6d5742425a629fc76e1b564a3c7781ca2a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122342
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Ben Clayton <bclayton@chromium.org>

src/tint/sem/index_accessor_expression.h

@@ -17,13 +17,9 @@
 
 #include <vector>
 
+#include "src/tint/ast/index_accessor_expression.h"
 #include "src/tint/sem/value_expression.h"
 
-// Forward declarations
-namespace tint::ast {
-class IndexAccessorExpression;
-}  // namespace tint::ast
-
 namespace tint::sem {
 
 /// IndexAccessorExpression holds the semantic information for a ast::IndexAccessorExpression node.
@@ -52,6 +48,11 @@ class IndexAccessorExpression final : public Castable<IndexAccessorExpression, V
     /// Destructor
     ~IndexAccessorExpression() override;
 
+    /// @returns the AST node
+    const ast::IndexAccessorExpression* Declaration() const {
+        return static_cast<const ast::IndexAccessorExpression*>(declaration_);
+    }
+
     /// @returns the object expression that is being indexed
     ValueExpression const* Object() const { return object_; }