c++: Update get_cxx_dialect_name for cxx29

Update get_cxx_dialect_name to handle cxx29 added by

commit 0c5c6249a2e9a75d2e7d1c6c434487c8dc077130
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Jun 16 22:24:57 2026 +0200

    c++, libcpp: Add -std=c++2[9d] and -std=gnu++2[9d] options

	PR c++/125851
	* name-lookup.cc (get_cxx_dialect_name): Handle cxx29.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>

gcc/cp/name-lookup.cc

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index e12e37c..60a6e6f 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -7555,6 +7555,8 @@
       return "C++23";
     case cxx26:
       return "C++26";
+    case cxx29:
+      return "C++29";
     }
 }