Fix crash and rejects-valid when a later template parameter or default

template argument contains a backreference to a dependently-typed
earlier parameter.

In a case like:
  template<typename T, T A, decltype(A) = A> struct X {};
  template<typename U> auto Y = X<U, 0>();
we previously treated both references to `A` in the third parameter as
being of type `int` when checking the template-id in `Y`. That`s wrong;
the type of `A` in these contexts is the dependent type `U`.

When we encounter a non-type template argument that we can't convert to
the parameter type because of type-dependence, we now insert a dependent
conversion node so that the SubstNonTypeTemplateParmExpr for the
template argument will have the parameter's type rather than whatever
type the argument had.

llvm-svn: 363972