Generating invalid code for aliases that do not use their template parameters

[fitzgen]

Simplest test case:

// bindgen-flags: -- --std=c++14
template <typename T>
using always_int = int;

Generates:

/* automatically generated by rust-bindgen */

pub type always_int<T> = ::std::os::raw::c_int;

Which rustc rejects with:

error[E0091]: type parameter `T` is unused
 --> /home/fitzgen/scratch/always_int.rs:3:21
  |
3 | pub type always_int<T> = ::std::os::raw::c_int;
  |                     ^ unused type parameter

[fitzgen]

This is fixed in #544