bring to dotty's GenBcode the fix for SI-3452 (Java generic signatures in mixins)

[magarciaEPFL]

The fix in question: scala/scala#3493

[retronym]

IMO static forwarders should be added before GenBCode. Then forwarders for inherited methods could be created with the "as-seen-from" signature, and erasure/post-erasure would take care of the primitive and derived value class boxing/unboxing.

[DarkDimius]

@odersky might be a good time to review this.

[DarkDimius]

I'm not going to work on this any time soon and this is low priority.

[DarkDimius]

the fix was fixing felixmulder#25

[odersky]

We also need someone to follow up on backend issues.

[SethTisue]

This falls under the category of aligning the Scala 2 and Scala 3 back ends. Is there still an open possibility of someone taking that category of issues on as a project?