libstdc++: Add a test checking for chrono::duration overflows

This test fails if chrono::days::rep or chrono::years::rep is a 32-bit
type, because a large days or years value silently overflows a 32-bit
integer when converted to seconds. It would be conforming to implement
chrono::days as chrono::duration<int32_t, ratio<86400>>, but would make
this overflow case more likely. Similarly for chrono::years,
chrono::months and chrono::weeks. This test is here to remind us not to
make that change lightly.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/duration/arithmetic/overflow_c++20.cc: New
	test.

libstdc++-v3/testsuite/20_util/duration/arithmetic/overflow_c++20.cc

+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+
+#include <chrono>
+#include <cstdint>
+#include <testsuite_hooks.h>
+
+void
+test_overflow()
+{
+  using namespace std::chrono;
+
+  using seconds32_t = duration<std::int_least32_t>;
+  seconds32_t t = 14h + 25min + 55s;
+  auto snow = sys_days(1854y/December/11);
+  auto snow_t = snow + t;
+  // Fails if days::rep is 32-bit:
+  VERIFY( snow_t.time_since_epoch() < seconds::zero() );
+  auto y = floor<years>(snow);
+  auto y_t = y + t;
+  // Fails if years::rep is 32-bit:
+  VERIFY( y_t.time_since_epoch() < seconds::zero() );
+  VERIFY( y_t < snow_t );
+}
+
+int main()
+{
+  test_overflow();
+}