Implement an _is_allocator type trait for use in deduction guides.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322306 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -5597,6 +5597,16 @@ struct __temp_value {
     };
 #endif
 
+#if _LIBCPP_STD_VER > 14
+template<typename _Alloc, typename = void>
+struct __is_allocator : false_type {};
+
+template<typename _Alloc>
+struct __is_allocator<_Alloc,
+     void_t<typename _Alloc::value_type, decltype(_VSTD::declval<_Alloc&>().allocate(size_t{}))>>
+   : true_type {};
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS