Aliases to template instantiations don't work

As alluded to here, libclang doesn't expose any API for us to get the arguments to the template instantiation when it is the aliased type of a type alias.

Input C/C++ Header

template <class T, class U>
struct Thing {
    T t;
    U u;
};

template <class V>
using IntThing = Thing<int, V>;

Bindgen Invocation

$ bindgen input.h -- -std=c++14

Actual Results

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Thing<T, U> {
    pub t: T,
    pub u: U,
    pub _phantom_0: ::std::marker::PhantomData<::std::cell::UnsafeCell<T>>,
    pub _phantom_1: ::std::marker::PhantomData<::std::cell::UnsafeCell<U>>,
}
pub type IntThing = Thing<T, U>;

Which fails to compile since T and U are not generic parameters in scope.

Expected Results

pub type IntThing<V> = Thing<int, V>;

Start with the linked context.rs location and reproduce the issue using the input.h template and bindgen input.h -- -std=c++14. Compare the generated IntThing alias with the expected generic alias, and consider the missing template arguments exposed by libclang. Done means the generated alias includes V and expands to Thing<int, V> without unresolved T or U.