ast: Make the fields of WorkgroupDecoration const

Change-Id: I76208ca1f6e3ccfb74c4732dfed8caddd4cec2ed
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60207
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/ast/workgroup_decoration.h b/src/ast/workgroup_decoration.h
index ab6d029..aaf9df9 100644
--- a/src/ast/workgroup_decoration.h
+++ b/src/ast/workgroup_decoration.h
@@ -64,9 +64,9 @@
   WorkgroupDecoration* Clone(CloneContext* ctx) const override;
 
  private:
-  ast::Expression* x_ = nullptr;
-  ast::Expression* y_ = nullptr;
-  ast::Expression* z_ = nullptr;
+  ast::Expression* const x_ = nullptr;
+  ast::Expression* const y_ = nullptr;
+  ast::Expression* const z_ = nullptr;
 };
 
 }  // namespace ast