Allow incomplete enum declarations

[krame505]

Low priority, but apparently GCC supports this as an extension: https://gcc.gnu.org/onlinedocs/gcc/Incomplete-Enums.html#Incomplete-Enums

Shouldn't be too hard to do if we decide to implement this at some point - we would just need to put the enum item in the refId namespace, and make some tweaks to the associated logic. This would also bring enum handling more inline with that of structs and unions.