tint: const eval of faceForward builtin

Bug: tint:1581
Change-Id: Ia50b4ec4d494face5e7f438037a092cd1f839849
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/111840
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/tint/resolver/resolver_test_helper.h b/src/tint/resolver/resolver_test_helper.h
index cf17b61..d44c9d8 100644
--- a/src/tint/resolver/resolver_test_helper.h
+++ b/src/tint/resolver/resolver_test_helper.h
@@ -185,7 +185,7 @@
 
 /// Returns current variant value in `s` cast to type `T`
 template <typename T>
-T As(Scalar& s) {
+T As(const Scalar& s) {
     return std::visit([](auto&& v) { return static_cast<T>(v); }, s);
 }