c++: alias CTAD and member alias templates [PR104470]

c++: alias CTAD and member alias templates [PR104470]

In this testcase, we were trying to substitute into
variant<Foo<T>>::__accepted_type, but failed to look it up because
variant<Foo<T>> doesn't exist.  In other cases we already rewrite such
things into a dependent reference; we need to do that for alias templates as
well.

This caused some testsuite regressions on alias uses outside of deduction
guides, so I've made all of this rewriting conditional on a new tf_dguide
tsubst flag.

	PR c++/104470

gcc/cp/ChangeLog:

	* cp-tree.h (enum tsubst_flags): Add tf_dguide.
	* pt.cc (tsubst_aggr_type): Check it.
	(tsubst_baselink, tsubst_copy): Check it.
	(maybe_dependent_member_ref): Check it.
	(instantiate_alias_template): Handle it.
	(build_deduction_guide): Set it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/explicit11.C: Second example also ill-formed.
	* g++.dg/cpp2a/class-deduction-alias12.C: New test.