Classes split over multiple files

I have the following situation (in many different situations):
```c++
// a.hpp
class A
{
class B;

void g(const B& b);
};

#include "b.hpp"
```

```c++
// b.hpp
class A::B
{
void g();
};
```

If I run hyde on `a.hpp` alone, I don't get any output for `B`. If I run hyde with with `b.hpp` alone or passing both `a.hpp` and `b.hpp`, I get:
```
.../hyde/build/in/b.hpp:1:7: error: use of undeclared identifier 'A'
class A::B
^
1 error generated.
```

Is there a way to support this use case?

Start by reproducing the issue with the a.hpp and b.hpp examples, then trace how hyde parses input files and handles declarations split across headers. Done means the nested class B is recognized when the files are processed together or separately without the undeclared A error, with documentation output generated for B.

Written by the indexing model from the issue text.