Fix -Werror=unused-but-set-variable

Change-Id: I17570776cce62eeef52a6667bc645388bd8597a6
Bug: dawn:1976
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/146682
Kokoro: Kokoro <[email protected]>
Reviewed-by: James Price <[email protected]>
Reviewed-by: Ryan Harrison <[email protected]>
Commit-Queue: Ben Clayton <[email protected]>

Author: sparist

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

@@ -2013,7 +2013,7 @@ Eval::Result Eval::ShiftRight(const core::type::Type* ty,
             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.