N3638 changes to return type deduction

N3638 changes to return type deduction

	* decl.c (undeduced_auto_decl): New.
	(require_deduced_type): New.
	(fndecl_declared_return_type): New.
	(decls_match): Use it.
	(duplicate_decls): Don't check for auto return.
	(grokdeclarator): Reject virtual auto.
	* class.c (resolve_address_of_overloaded_function): Handle
	auto function templates.
	* decl2.c (mark_used): Use undeduced_auto_decl, require_deduced_type.
	* cp-tree.h: Declare new fns.
	* error.c (dump_function_decl): Use fndecl_declared_return_type.
	* search.c (check_final_overrider): Likewise.
	* pt.c (make_decltype_auto): New.
	(do_auto_deduction): Require plain decltype(auto).
	(is_auto): Adjust.

From-SVN: r198099