Fix -Werror=unused-but-set-variable

Change-Id: I17570776cce62eeef52a6667bc645388bd8597a6
Bug: dawn:1976
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/146682
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/lang/core/constant/eval.cc

diff --git a/src/tint/lang/core/constant/eval.cc b/src/tint/lang/core/constant/eval.cc
index 0da84aa..44b24e1 100644
--- a/src/tint/lang/core/constant/eval.cc
+++ b/src/tint/lang/core/constant/eval.cc
@@ -2013,7 +2013,7 @@
             UT e1u = static_cast<UT>(e1);
             UT e2u = static_cast<UT>(e2);
 
-            auto signed_shift_right = [&] {
+            [[maybe_unused]] auto signed_shift_right = [&] {
                 // In C++, right shift of a signed negative number is implementation-defined.
                 // Although most implementations sign-extend, we do it manually to ensure it works
                 // correctly on all implementations.