[libc++][nfc] Only test if pair is_assignable after C++03.

In C++03 libc++ uses a different set of constructors which aren't
constrained, so these tests won't work. This should fix the bots.

Refs: 82c4701.

Author: zoecarver

libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_const_pair_U_V.pass.cpp

@@ -63,8 +63,6 @@ TEST_CONSTEXPR_CXX20 bool test() {
     assert(p.first == 42);
     assert(p.second.value == -42);
   }
-#endif
-
   { // test const requirement
     using T = std::pair<CopyAssignableInt, CopyAssignableInt>;
     using P = std::pair<int, int>;
@@ -76,6 +74,7 @@ TEST_CONSTEXPR_CXX20 bool test() {
     static_assert(!std::is_assignable<T&, P&>::value, "");
     static_assert(!std::is_assignable<P&, T&>::value, "");
   }
+#endif
   return true;
 }