Ensure that cuda::std::pair is potentially trivially copyable

trivially copyable is a requirement for memcpy. We want to ensure that our pair implementation satisfies that whenever possible.

This is especially important for thrust::pair as that is used in rmm extensively.

Fixes #1246

libcudacxx/include/cuda/std/detail/libcxx/include/__concepts/__concept_macros.h

@@ -26,8 +26,6 @@
 #  pragma system_header
 #endif // no system header
 
-#if _CCCL_STD_VER > 2011
-
 #define _LIBCUDACXX_PP_CAT_(_Xp, ...) _Xp##__VA_ARGS__
 #define _LIBCUDACXX_PP_CAT(_Xp, ...) _LIBCUDACXX_PP_CAT_(_Xp, __VA_ARGS__)
 
@@ -285,7 +283,7 @@ _LIBCUDACXX_INLINE_VISIBILITY inline constexpr bool _Is_true() {
   return true;
 }
 
-#if defined(_LIBCUDACXX_COMPILER_CLANG) || defined(_LIBCUDACXX_COMPILER_MSVC)
+#if defined(_LIBCUDACXX_COMPILER_CLANG) || defined(_LIBCUDACXX_COMPILER_MSVC) || _CCCL_STD_VER == 2011
 template <bool _Bp>
 _LIBCUDACXX_INLINE_VISIBILITY _Concept::_Enable_if_t<_Bp> _Requires() {}
 #else
@@ -294,6 +292,4 @@ _LIBCUDACXX_INLINE_VAR constexpr int _Requires = 0;
 #endif
 } // namespace _Concept
 
-#endif // _CCCL_STD_VER > 2011
-
 #endif //_CUDA___CONCEPTS