Replace the approach of “detect some magic combination of flags to
trigger some action” by introducing an enum flag which makes the
semantics and the intentions of the code using it more explicit.
This basically involves wiring up the existing ACC_ENUM bit to a new
modifier flag and making sure it is set correctly when encountering
This will enable us to treat enums more consistently and will allow us to support the declaration of enums directly in Scala (e. g. via macro annotations).