c++: alias template and typename [PR103057]

Usually we handle DR1558 substitution near the top of tsubst, but in this
case while substituting TYPENAME_TYPE we were passing an alias
specialization to tsubst_aggr_type, which ignored its aliasness.

	PR c++/103057

gcc/cp/ChangeLog:

	* pt.cc (tsubst_aggr_type): Call tsubst for alias template
	specialization.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-void1.C: New test.