c++: ICE with alias in pack expansion [PR103769]

This was breaking because when we stripped the 't' typedef in s<t<Args>...>
to be s<Args...>, the TYPE_MAIN_VARIANT of "Args..." was still
"t<Args>...", because type pack expansions are treated as types.  Fixed by
using the right function to copy a "type".

	PR c++/99445
	PR c++/103769

gcc/cp/ChangeLog:

	* tree.cc (strip_typedefs): Use build_distinct_type_copy.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/variadic-alias5.C: New test.