c++: Simplify typedef access checking

c++: Simplify typedef access checking

I discovered that the template typedef access check was rather more
expensive than needed.  The call of get_types_needed_access_check in
the FOR_EACH_VEC_SAFE_ELT is the moral equivalent of 'for (size_t pos
= 0; pos != strlen (string); pos++)'.  Let's not do that.

	* pt.c (perform_typedefs_access_check): Cache expensively
	calculated object references.
	(check_auto_in_tmpl_args): Just assert we do not get unexpected
	nodes, rather than silently do nothing.
	(append_type_to_template_for_access): Likewise, cache expensie
	object reference.