c: Fix ICE in c_type_tag on va_list [PR121506]

The C and C++ FEs disagree on what TYPE_NAME on RECORD_TYPE for
structure/class definition is (rather than typedef/using, for
those both have TYPE_NAME of TYPE_DECL with DECL_ORIGINAL_TYPE),
the C FE just uses IDENTIFIER_NODE as TYPE_NAME on RECORD_TYPE,
while the C++ FE uses TYPE_DECL as TYPENAME on RECORD_TYPE and
only DECL_NAME on the TYPE_DECL provides the IDENTIFIER_NODE.
The reason for the C++ FE way is that there can be type definitions
at class scope (rather than just typedefs) and those need to be
among TYPE_FIELDS (so the corresponding TYPE_DECL is in that chain)
etc.
The middle-end can cope with it, e.g.
                if (TREE_CODE (TYPE_NAME (node)) == IDENTIFIER_NODE)
                  pp_tree_identifier (pp, TYPE_NAME (node));
                else if (TREE_CODE (TYPE_NAME (node)) == TYPE_DECL
                         && DECL_NAME (TYPE_NAME (node)))
                  dump_decl_name (pp, TYPE_NAME (node), flags);
and many other places.
Now, the backends on various targets create artificial structure
definitions for va_list, e.g. x86 creates struct __va_list_tag
and they do it the C++ FE way so that the C++ FE can cope with those.
Except the new c_type_tag can't deal with that and ICEs.

The following patch fixes it so that it can handle it too.

2025-11-27  Jakub Jelinek  <[email protected]>

	PR c/121506
	* c-typeck.cc (c_type_tag): If TYPE_NAME is TYPE_DECL
	with non-NULL DECL_NAME, return that.

	* gcc.dg/pr121506.c: New test.

Author: jakubjelinek

gcc/c/c-typeck.cc

@@ -610,9 +610,9 @@ c_type_tag (const_tree t)
     return NULL_TREE;
   if (TREE_CODE (name) == TYPE_DECL)
     {
-      /* A TYPE_DECL added by add_decl_expr.  */
-      gcc_checking_assert (!DECL_NAME (name));
-      return NULL_TREE;
+      if (!DECL_NAME (name))
+	return NULL_TREE;
+      name = DECL_NAME (name);
     }
   gcc_checking_assert (TREE_CODE (name) == IDENTIFIER_NODE);
   return name;

gcc/testsuite/gcc.dg/pr121506.c

@@ -0,0 +1,8 @@
+/* PR c/121506 */
+/* { dg-do compile } */
+
+#include <stdarg.h>
+
+struct A;
+void foo (struct A *);	/* { dg-message "previous declaration of 'foo' with type 'void\\\(struct A \\\*\\\)'" } */
+void foo (va_list);	/* { dg-error "conflicting types for 'foo'; have" } */