C++: Enable c++2b module mode [PR 99436]

[Nathan Sidwell]

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

diff --git c/gcc/cp/name-lookup.c w/gcc/cp/name-lookup.c
index 092fa6b8768..28271ba0485 100644
--- c/gcc/cp/name-lookup.c
+++ w/gcc/cp/name-lookup.c
@@ -6963,6 +6963,8 @@ get_cxx_dialect_name (enum cxx_dialect dialect)
       return "C++17";
     case cxx20:
       return "C++20";
+    case cxx23:
+      return "C++23";
     }
 }
 
diff --git c/gcc/testsuite/g++.dg/modules/modules.exp w/gcc/testsuite/g++.dg/modules/modules.exp
index 38654caf7b9..da7afc2a226 100644
--- c/gcc/testsuite/g++.dg/modules/modules.exp
+++ w/gcc/testsuite/g++.dg/modules/modules.exp
@@ -35,7 +35,7 @@ if ![info exists DEFAULT_CXXFLAGS] then {
     set DEFAULT_CXXFLAGS " -pedantic-errors -Wno-long-long"
 }
 set DEFAULT_MODFLAGS $DEFAULT_CXXFLAGS
-set MOD_STD_LIST { 17 2a }
+set MOD_STD_LIST { 17 2a 2b }
 
 dg-init