jira bugs to squash for rc1

[lrytz]

lukas:

• https://issues.scala-lang.org/browse/SI-8786 (SI-8786 fix generic signature for @varargs forwarder methods scala#5262)
• https://issues.scala-lang.org/browse/SI-8561 (SI-8561 named subclasses for known Manifest / ClassTag instances scala#5264)
• https://issues.scala-lang.org/browse/SI-5289 – out of scope, see comments on ticket