Correctly handle packs for variadic type traits.

I'm not sure how to write a test for this; the following shows the
difference in -ast-dump:

template <int x> struct A {};
template <class T> struct B { };
template <class ...Args> using C = A<(__is_trivially_constructible(Args...))>;
template <class ...Args> using D = C<B<Args>...>;

However, I can't seem to write a test that triggers a visible difference
in behavior.