Better support for enum trees

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
enums.

The existing enum tests files/pos/t5165 and files/pos/t2764 keep
working, showing that this hasn't introduced any obvious regressions.

Tests for the changes in Namer which prevent scalac from messing with
enum trees can't be added yet, because one currently can't define an
enum in Scala without the help of the macro paradise.
The intention is to add the @enum macro as a full test suite as soon
as one can depend on macro annotations.