Take care of all the missing outer pointer in Scala 2 library

[hamzaremmal]

Now that SIP-52 was approved, we can use it to hide constructors that would have been generated by nsc but not dotc.

[sjrd]

I don't understand why we need these @publicInBinary definitions. All the introduced constructors are in private classes. That means no call to those constructors could ever exist in another compilation unit.

IMO for these cases we should keep the MiMa filters instead, with a comment for that section saying "constructors of private classes, not an issue".

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For the two private val _ = Enclosing.this in objects, that seems warranted. However :

1. It would be good to first have a commit with a verbatim copy of the file upstream.
2. Then a commit that actually introduces the patch and removes the MiMa filter.

Otherwise it's very tricky to determine now, and in the future, what changed.

Moreover, these lines really deserve comments explaining why they are there.