Test and fix for 1289 ("Support for anonymous enums")

[badicsalex]

Test and draft fix for #1289

The actual fix patch is very crude and I doubt this is the best way to do it, but it makes the tests pass at least, and I didn't have that much time to go through the autocxx source.

• Tests pass
• Appropriate changes to README are included in PR

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[adetaylor]

Thanks a ton for the test case and the attempted fix. Let's see what happens on the existing test suite!

[adetaylor]

Ideally this would generate an error message at a later analysis phase, so that people get rustdoc explaining what went wrong, but this is a good start, thanks!

[badicsalex]

Nice. This was super quick, thanks.