c++: fix ICE on invalid attributes [PR96637]

c++: fix ICE on invalid attributes [PR96637]

When chaining attributes, attr_chainon should be used rather than plain
chainon, so that we don't end up with a TREE_LIST where one of the elements
is error_mark_node, which causes problems.  parser.cc has already been
fixed to use attr_chainon, but decl.cc has not.  Until now.

	PR c++/96637

gcc/cp/ChangeLog:

	* cp-tree.h (attr_chainon): Declare.
	* decl.cc (start_decl): Use attr_chainon.
	(grokdeclarator): Likewise.
	* parser.cc (cp_parser_statement): No longer static.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/error64.C: New test.