c++: ICE when substituting packs into type aliases [PR118104]

c++: ICE when substituting packs into type aliases [PR118104]

r12-1094 mentions that adding the assert didn't lead to any regressions
in the testsuite, but this test case demonstrates that we can reach it
with valid code.

Here we arrive in use_pack_expansion_extra_args_p with t which is an
expansion whose pattern is void(Ts, Us) and tparm packs are {Us, Ts},
and parm_packs is { Ts -> <int, int>, Us -> <A, P...> }.  We want to
expand the pack into void(int, A) and void(int, P...).  We compare
int to A, which is fine, but then int to P... which crashes.  But
the code is valid so this patch removes the assert.

	PR c++/118104

gcc/cp/ChangeLog:

	* pt.cc (use_pack_expansion_extra_args_p): Remove an assert.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-variadic3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 6751bd4ac48a8529e2476a6848a77e81de540814)