[libcxx][test] Use long double test macro in strong_order.pass.cpp (llvm#106742)

Author: DavidSpickett

libcxx/test/std/language.support/cmp/cmp.alg/strong_order.pass.cpp

@@ -454,12 +454,16 @@ int main(int, char**)
     test_1_2();
     test_1_3<float>();
     test_1_3<double>();
-    // test_1_3<long double>();  // UNIMPLEMENTED
+#ifdef TEST_LONG_DOUBLE_IS_DOUBLE
+    test_1_3<long double>(); // UNIMPLEMENTED when long double is a distinct type
+#endif
     test_1_4();
 
     static_assert(test_1_3<float>());
     static_assert(test_1_3<double>());
-    // static_assert(test_1_3<long double>());  // UNIMPLEMENTED
+#ifdef TEST_LONG_DOUBLE_IS_DOUBLE
+    static_assert(test_1_3<long double>()); // UNIMPLEMENTED when long double is a distinct type
+#endif
     static_assert(test_1_4());
 
     return 0;