Fix GCC warning about an unused local variable

Change-Id: Idd064713ce8919e96f84a8a13fa04779fa889208
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44871
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/writer/hlsl/generator_impl.cc b/src/writer/hlsl/generator_impl.cc
index 675f401..186ed20 100644
--- a/src/writer/hlsl/generator_impl.cc
+++ b/src/writer/hlsl/generator_impl.cc
@@ -2594,7 +2594,7 @@
           return false;
         }
         out << " : " << attr;
-      } else if (auto* offset = deco->As<ast::StructMemberOffsetDecoration>()) {
+      } else if (deco->Is<ast::StructMemberOffsetDecoration>()) {
         // Nothing to do, offsets are handled at the point of access.
       } else {
         diagnostics_.add_error("unsupported struct member decoration");