[libc++] Don't declare pmr::polymorphic_allocator before C++17

Author: philnik777

libcxx/include/__fwd/memory_resource.h

@@ -15,6 +15,8 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 17
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace pmr {
@@ -24,4 +26,6 @@ class _LIBCPP_AVAILABILITY_PMR _LIBCPP_TEMPLATE_VIS polymorphic_allocator;
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 17
+
 #endif // _LIBCPP___FWD_MEMORY_RESOURCE_H