c++: Handle enumerator in C++20 alias CTAD. [PR96199]

c++: Handle enumerator in C++20 alias CTAD. [PR96199]

To form a deduction guide for an alias template, we substitute the template
arguments from the pattern into the deduction guide for the underlying
class.  In the case of B(A1<X>), that produces B(A1<B<T,1>::X>) -> B<T,1>.
But since an enumerator doesn't have its own template info, and B<T,1> is a
dependent scope, trying to look up B<T,1>::X fails and we crash.  So we need
to produce a SCOPE_REF instead.

And trying to use the members of the template class is wrong for other
members, as well, as it gives a nonsensical result if the class is
specialized.

gcc/cp/ChangeLog:

	PR c++/96199
	* pt.c (maybe_dependent_member_ref): New.
	(tsubst_copy) [CONST_DECL]: Use it.
	[VAR_DECL]: Likewise.
	(tsubst_aggr_type): Handle nested type.

gcc/testsuite/ChangeLog:

	PR c++/96199
	* g++.dg/cpp2a/class-deduction-alias4.C: New test.