not be a pack expansion type.
Using a pack expansion type for a pack declaration makes sense, but
general expressions should never have pack expansion types. If we have a
pack `T *...V`, then the type of `V` is the type `T *`, which contains
an unexpanded pack, and is a pointer type.
This allows us to better diagnose issues where a template is invalid due
to some non-dependent portion of a dependent type of a non-type template
parameter pack.
