c++: alias CTAD refactoring [PR104470]

c++: alias CTAD refactoring [PR104470]

In my previous PR104470 patch I added yet another place that needs to handle
dependent member rewriting for deduction guides; this patches centralizes
rewriting into maybe_dependent_member_ref.  tsubst_baselink still has its
own handling because that's simpler than teaching maybe_dependent_member_ref
about BASELINKs.

	PR c++/104470

gcc/cp/ChangeLog:

	* pt.cc (maybe_dependent_member_ref): Handle types.
	(tsubst, tsubst_copy): Use it.
	(tsubst_aggr_type, instantiate_alias_template): Don't handle
	tf_dguide here.