c++: Fix ICE-on-invalid with -Wvexing-parse [PR97881]

This invalid (?) code broke my assumption that if decl_specifiers->type
is null, there must be any type-specifiers.  Turn the assert into an if
to fix this crash.

gcc/cp/ChangeLog:

	PR c++/97881
	* parser.c (warn_about_ambiguous_parse): Only assume "int" if we
	actually saw any type-specifiers.

gcc/testsuite/ChangeLog:

	PR c++/97881
	* g++.dg/warn/Wvexing-parse9.C: New test.

gcc/cp/parser.c

@@ -20790,13 +20790,12 @@ warn_about_ambiguous_parse (const cp_decl_specifier_seq *decl_specifiers,
       if (same_type_p (type, void_type_node))
 	return;
     }
+  else if (decl_specifiers->any_type_specifiers_p)
+    /* Code like long f(); will have null ->type.  If we have any
+       type-specifiers, pretend we've seen int.  */
+    type = integer_type_node;
   else
-    {
-      /* Code like long f(); will have null ->type.  If we have any
-	 type-specifiers, pretend we've seen int.  */
-      gcc_checking_assert (decl_specifiers->any_type_specifiers_p);
-      type = integer_type_node;
-    }
+    return;
   auto_diagnostic_group d;
   location_t loc = declarator->u.function.parens_loc;

gcc/testsuite/g++.dg/warn/Wvexing-parse9.C

+// PR c++/97881
+// { dg-do compile }
+
+void
+cb ()
+{
+  volatile _Atomic (int) a1; // { dg-error "" }
+}