Assimp (asset import library) crashes code generator

```
Traceback (most recent call last):
File "/home/user/sandboxed-api/sandboxed_api/tools/generator2/sapi_generator.py", line 73, in
app.run(main)
File "/usr/lib/python3.10/site-packages/absl/app.py", line 312, in run
_run_main(main, args)
File "/usr/lib/python3.10/site-packages/absl/app.py", line 258, in _run_main
sys.exit(main(argv))
File "/home/user/sandboxed-api/sandboxed_api/tools/generator2/sapi_generator.py", line 60, in main
result = generator.generate(FLAGS.sapi_name, FLAGS.sapi_functions,
File "/home/user/sandboxed-api/sandboxed_api/tools/generator2/code.py", line 729, in generate
related_types = self._get_related_types(function_names)
File "/home/user/sandboxed-api/sandboxed_api/tools/generator2/code.py", line 790, in _get_related_types
types += sorted(r for r in fn_related_types if r not in processed)
File "/home/user/sandboxed-api/sandboxed_api/tools/generator2/code.py", line 165, in __gt__
self._tu.order[other._get_declaration().hash]) # pylint: disable=protected-access
KeyError: 3995101709
```

The key depends on the set of headers chosen. Not all of the headers cause this, but many do.