This adds support for c++23 mode to modules, and enables such testing.

	PR c++/99436
         gcc/cp/
         * name-lookup.c (get_cxx_dialect_name):	Add cxx23.
         gcc/testsuite/
         * g++.dg/modules/modules.exp (MOD_STD_LIST): Add 2b.

-- 
Nathan Sidwell