PR c++/93286 - ICE with __is_constructible and variadic template.

Here we had been recursing in tsubst_copy_and_build if type2 was a TREE_LIST
because that function knew how to deal with pack expansions, and tsubst
didn't.  But tsubst_copy_and_build expects to be dealing with expressions,
so we crash when trying to convert_from_reference a type.

	* pt.c (tsubst) [TREE_LIST]: Handle pack expansion.
	(tsubst_copy_and_build) [TRAIT_EXPR]: Always use tsubst for type2.