move implementation of ordinal in enums to posttyper

[bishabosha]

It was identified by @odersky that the cause of #13554 was forcing of the enum class in desugaring (in isJavaEnum method). isJavaEnum is only used to provide an implementation of ordinal for singleton cases, so the whole thing can be deferred until later.

also add a check that ordinal is not implemented by the user in the body of an enum class, or mixed in by a trait - this is necessary as scala.deriving.Mirror.Sum delegates to ordinal method on an enum.

Note- before this PR enum cases would declare ordinal methods without an override flag, so refchecks would issue an override-without-override-modifier error anyway if the user declared their own.

fixes #13554

[bishabosha]

getting this error in community build:

Error:  -- [E069] Syntax Error: /__w/dotty/dotty/library/src/scala/IArray.scala:13:7 ---
Error:  13 |object IArray:
Error:     |       ^
Error:     |object IArray cannot have the same name as object IArray in package scala -- cannot define object member with the same name as a object member in self reference _.
Error:     |(Note: this can be resolved by using another name)

[odersky]

@bishabosha Moving things from desugar to post typer completely changes the rules of the game, and is not easy either. What you got is a hard problem which means that things get completed too early. I maintain my earlier sentiment that the tradeoff between benefit and complexity/fragility of this feature is negative.

[smarter]

Error: |object IArray cannot have the same name as object IArray in package scala -- cannot define object member with the same name as a object member in self reference _.

I don't think this is related to this PR, this error already happens on master but is ignored, I think it happens when scaladoc unpickles some classfile.

[bishabosha]

@odersky we need to implement ordinal but only when we do not extend java.lang.Enum (as ordinal is final in that class) - so I propose the other solution is to keep adding ordinal in desugaring, and delete the ordinal method from Java enums before TASTy/zinc phases

[michelou]

@smarter Better error messages in neg tests 👍