c++: Fix member alias template in C++17 and up. [PR96805]

c++: Fix member alias template in C++17 and up. [PR96805]

Here we're trying to push into a<T>::c<N> in order to instantiate t<N>, but
were building a TYPENAME_TYPE for it because a<T> isn't open yet.  Don't
do that when we know we're trying to enter the scope.

gcc/cp/ChangeLog:

	PR c++/96805
	PR c++/96199
	* pt.c (tsubst_aggr_type): Don't build a TYPENAME_TYPE when
	entering_scope.
	(tsubst_template_decl): Use tsubst_aggr_type.

gcc/testsuite/ChangeLog:

	PR c++/96805
	* g++.dg/cpp0x/alias-decl-pr96805.C: New test.