PR c++/96440: ICE-on-invalid-code error recovery.

This patch fixes PR c++/96440 which is an ICE-on-invalid-code regression
affecting mainline.

2022-03-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/cp/ChangeLog
	PR c++/96440
	* decl.cc (start_decl): Defend against prefix_attributes being
	error_mark_node.

gcc/testsuite/ChangeLog
	PR c++/96440
	* g++.dg/cpp0x/pr96440.C: New test case.

gcc/cp/decl.cc

@@ -5483,13 +5483,15 @@ start_decl (const cp_declarator *declarator,
   *pushed_scope_p = NULL_TREE;
-  attributes = chainon (attributes, prefix_attributes);
+  if (prefix_attributes != error_mark_node)
+    attributes = chainon (attributes, prefix_attributes);
   decl = grokdeclarator (declarator, declspecs, NORMAL, initialized,
 			 &attributes);
   if (decl == NULL_TREE || VOID_TYPE_P (decl)
-      || decl == error_mark_node)
+      || decl == error_mark_node
+      || prefix_attributes == error_mark_node)
     return error_mark_node;
   context = CP_DECL_CONTEXT (decl);

gcc/testsuite/g++.dg/cpp0x/pr96440.C

+/* { dg-do compile } */
+/* { dg-excess-errors "" } */
+
+auto alignas a [[]] [[const]] ();