[libc++] Properly guard flat_map includes based on C++ version

That's what we (try to) do consistently for all other umbrella headers.

Author: ldionne

libcxx/include/flat_map

@@ -39,8 +39,13 @@ namespace std {
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <__flat_map/flat_map.h>
-#include <__flat_map/sorted_unique.h>
+
+#if _LIBCPP_STD_VER >= 23
+#  include <__flat_map/flat_map.h>
+#  include <__flat_map/sorted_unique.h>
+#endif
+
+// for feature-test macros
 #include <version>
 
 // standard required includes