c++: hash table ICE with variadic alias [PR105003]

For PR104008 we thought it might be enough to keep strip_typedefs from
removing this alias template specialization, but this PR demonstrates that
other parts of the compiler also need to know to consider it dependent.

So, this patch changes complex_alias_template_p to no longer consider
template parameters used when their only use appears in a pack expansion,
unless they are the parameter packs being expanded.

To do that I also needed to change it to use cp_walk_tree instead of
for_each_template_parm.  It occurs to me that find_template_parameters
should probably also use cp_walk_tree, but I'm not messing with that now.

	PR c++/105003
	PR c++/104008
	PR c++/102869

gcc/cp/ChangeLog:

	* pt.cc (complex_alias_template_r): walk_tree callback,	replacing
	uses_all_template_parms_r, complex_pack_expansion_r.
	(complex_alias_template_p): Adjust.
	* tree.cc (strip_typedefs): Revert r12-7710 change.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/variadic-alias6.C: New test.
	* g++.dg/cpp0x/variadic-alias7.C: New test.