[libc++] Improve coverage of std::atomic_ref<T>::exchange() (#121596)

Adapted from libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/exchange.pass.cpp
as we did for testing other functionalities. Spotted that lapse in coverage when working on #121414.

Author: dalg24

libcxx/test/std/atomics/atomics.ref/exchange.pass.cpp

@@ -17,24 +17,47 @@
 #include <type_traits>
 
 #include "atomic_helpers.h"
+#include "test_helper.h"
 #include "test_macros.h"
 
 template <typename T>
 struct TestExchange {
   void operator()() const {
-    T x(T(1));
-    std::atomic_ref<T> const a(x);
+    {
+      T x(T(1));
+      std::atomic_ref<T> const a(x);
+
+      {
+        std::same_as<T> decltype(auto) y = a.exchange(T(2));
+        assert(y == T(1));
+        ASSERT_NOEXCEPT(a.exchange(T(2)));
+      }
+
+      {
+        std::same_as<T> decltype(auto) y = a.exchange(T(3), std::memory_order_seq_cst);
+        assert(y == T(2));
+        ASSERT_NOEXCEPT(a.exchange(T(3), std::memory_order_seq_cst));
+      }
+    }
 
+    // memory_order::release
     {
-      std::same_as<T> decltype(auto) y = a.exchange(T(2));
-      assert(y == T(1));
-      ASSERT_NOEXCEPT(a.exchange(T(2)));
+      auto exchange = [](std::atomic_ref<T> const& x, T, T new_val) {
+        x.exchange(new_val, std::memory_order::release);
+      };
+      auto load = [](std::atomic_ref<T> const& x) { return x.load(std::memory_order::acquire); };
+      test_acquire_release<T>(exchange, load);
     }
 
+    // memory_order::seq_cst
     {
-      std::same_as<T> decltype(auto) y = a.exchange(T(3), std::memory_order_seq_cst);
-      assert(y == T(2));
-      ASSERT_NOEXCEPT(a.exchange(T(3), std::memory_order_seq_cst));
+      auto exchange_no_arg     = [](std::atomic_ref<T> const& x, T, T new_val) { x.exchange(new_val); };
+      auto exchange_with_order = [](std::atomic_ref<T> const& x, T, T new_val) {
+        x.exchange(new_val, std::memory_order::seq_cst);
+      };
+      auto load = [](std::atomic_ref<T> const& x) { return x.load(); };
+      test_seq_cst<T>(exchange_no_arg, load);
+      test_seq_cst<T>(exchange_with_order, load);
     }
   }
 };