Fixes for nodiscard std::optional, std::set operations

libc++ recently added more nodiscard attributes:
http://github.com/llvm/llvm-project/pull/169982 and
http://github.com/llvm/llvm-project/pull/170045

Bug: 468205004
Change-Id: Ie199d266b349696515e17bf8735333a202134f51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7256034
Owners-Override: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1557956}
NOKEYCHECK=True
GitOrigin-RevId: 54f527e0399c84e1d70fdeb17b4fa8d53b4bf8f3
diff --git a/absl_hardening_test.cc b/absl_hardening_test.cc
index 6c4fcb1..3d2ca55 100644
--- a/absl_hardening_test.cc
+++ b/absl_hardening_test.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include <limits>
+#include <utility>
 #include <vector>
 
 #include "gtest/gtest.h"
@@ -22,7 +23,7 @@
 
 TEST(AbslHardeningTest, Optional) {
   absl::optional<int> optional;
-  EXPECT_DEATH_IF_SUPPORTED(*optional, "");
+  EXPECT_DEATH_IF_SUPPORTED(std::ignore = *optional, "");
 }
 
 TEST(AbslHardeningTest, StringView) {