[RFC] Follow-up on doctrine/collections Order enum

@derrabus

In doctrine/collections#389, I introduced a new enum: Order

What should we use it for inside the ORM?

The OrderBy attribute: this is a clear use case where we should switch to that enum, since it has to do with collections
QueryBuilder::orderBy() / addOrderBy(): this one is less clear to me… when you select several entities, you don't get a collection, but a list, right?
DQL : I don't think we should allow it inside DQL, it's cumbersome
Expr\OrderBy: I'm not sure we should do it here either.

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