Classes with external linkage should be defined in a header file

This is just an extension of Effectively, classes should either be defined in a header file, or should be defined in an unnamed namespace.

Hi @torgeir.skogen,

Thanks for sharing this idea.

I created [CPP-4029] - Jira to investigate it further. FWIW, since the initial rule is based on MISRA, it makes sense for us to consider the next version of MISRA before implementing this new rule and any corner case, if any.