filter out varargs star more uniformly

[scabug]

In scala/scala@649c2784 I modified lub not to accidentally lub its way to T* types, and included this comment:

-      existentialAbstraction(tparams, lubType)
+      // dropRepeatedParamType is a localized fix for SI-6897. We should probably
+      // integrate that transformation at a lower level in master, but lubs are
+      // the likely and maybe only spot they escape, so fixing here for 2.10.1.
+      existentialAbstraction(tparams, dropRepeatedParamType(lubType))

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6929?orig=1
Reporter: @paulp
See #6897

[SethTisue]

#8923 is in the same area.

@som-snytt the issue description here is vague. I'm tentatively closing on the presumption that after scala/scala#7399, more fundamental changes in this area are unlikely in Scala 2, but we can reopen if there's dissent