Refactor/simplify DtoCallFunction().

* No need to keep the argiter iterator up-to-date, simply make use of
  args.size().
* Simplify signature of addTypeinfoArrayArgumentForDVarArg().
* Don't perform casts to void* for arguments to vararg intrinsics here.
  TargetABI now takes care of this.
* Simplify handling of reversed parameter order by allocating all
  elements first and then simply indexing.