Use some enums in compiler

[odersky]

No description provided.

[odersky]

There are actually surprisingly few opportunities to use enums. The problem is that most simple enumerations we use in the compiler have bitset structure. So an enum does not work for them, at least
not in a straightforward way.

[nicolasstucki]

The last one that could be a Scala enum is dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

[dwijnand]

Is it not possible to add derives Ordering to those enums, to avoid the .ordinals?

[nicolasstucki]

We also need stable IDs which an ordering would not give.

[dwijnand]

I was hoping it would use the enum's ordinal?

still doesn't know how derives works or should be used...

[odersky]

We could use a derived ordering for these. But for the moment using a couple .ordinals seems OK. It's not so high in priority.

[odersky]

The remaining failures have to do with -Ytest-pickler instabilities of positions of Child annotations (fixed in #6752). I'm putting this PR on hold until the next bootstrap compiler is out that fixes the problem.

[anatoliykmetyuk]

ListBuffer[ValDef] is unambiguous. Meaning there is no question whether it is a reference to an object or a constructor call here because of the type parameters. Can we drop () here in future?