backport: re PR c++/83824 (ICE on invalid C++ code with alignas: in chainon, at tree...

backport: re PR c++/83824 (ICE on invalid C++ code with alignas: in chainon, at tree.c:3037)

	Backported from mainline
	2018-01-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/83824
	* parser.c (attr_chainon): New function.
	(cp_parser_label_for_labeled_statement, cp_parser_decl_specifier_seq,
	cp_parser_namespace_definition, cp_parser_init_declarator,
	cp_parser_type_specifier_seq, cp_parser_parameter_declaration,
	cp_parser_gnu_attributes_opt): Use it.
	(cp_parser_member_declaration, cp_parser_objc_class_ivars,
	cp_parser_objc_struct_declaration): Likewise.  Don't reset
	prefix_attributes if attributes is error_mark_node.

	* g++.dg/cpp0x/pr83824.C: New test.

From-SVN: r262054