Tagged unions inside template fail to build.

This is sort of a rustc limitation, but we could try to detect it better.

Input C/C++ Header

template <typename _Tp, typename _Alloc>
struct _Vector_base {
  union _Storage {
    constexpr _Storage() : _M_byte() {}
    ~_Storage() {}
    _Storage& operator=(const _Storage&) = delete;
    unsigned char _M_byte;
    _Tp _M_val;
  };
};

Bindgen Invocation

$ bindgen input.hpp | rustc -

Actual Results

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct _Vector_base {
    pub _address: u8,
}
#[repr(C)]
pub union _Vector_base__Storage<_Tp> {
    pub _M_byte: ::std::os::raw::c_uchar,
    pub _M_val: _Tp,
    pub _phantom_0: ::std::marker::PhantomData<::std::cell::UnsafeCell<_Tp>>,
}

and/or

error[E0740]: unions may not contain fields that need dropping
  --> <anon>:11:5
   |
11 |     pub _M_val: _Tp,
   |     ^^^^^^^^^^^^^^^
   |
help: wrap the type with `std::mem::ManuallyDrop` and ensure it is manually dropped
   |
11 |     pub _M_val: std::mem::ManuallyDrop<_Tp>,
   |                 +++++++++++++++++++++++   +

error: aborting due to 2 previous errors

Expected Results

We should probably generate non-rust unions in this case, or add ManuallyDrop around all the types in a union or something like that.

Start with the input.hpp template and the generated _Vector_base__Storage union, then reproduce the failure with bindgen input.hpp | rustc -. Determine how bindgen handles template unions containing potentially droppable fields; done means the reported input is detected or generated bindings compile without the union error.