Fix error when using Sphinx renderer with C# enums

[Esvandiary]

Fixes #847

The error appears to be related to Doxygen applying strong="yes" to the enums' memberdef tags in the XML, as seen in the example input here. This results in the Sphinx renderer marking their object type as enum-class rather than enum; however, there is no enum-class object type defined in the C# type mapping, so an exception occurs.

This PR takes the route of defining enum-class in the C# type mapping with an identical definition to enum; this seems like the least disruptive option, but I don't know if a different solution (e.g. checking if we're generating for C# when deciding the object type and not using the enum-class type if so) would be preferred.

[jakobandersen]

Thanks for reporting and for the PR. I find it a bit haxy to pretend a directive exists when it doesn't, so I have made #849 which fixes it in the way you allude to. Can you double-check that it fixes things on your end as well?

[Esvandiary]

Agreed that #849 is much cleaner - I just wasn't familiar enough with the codebase to know how to achieve it 😄
I can confirm it also fixes my problem, so quite happy to close this in favour of your implementation!