SI-8283 mutation-free bound inference for existentials

A safer version of the fix for SI-6169 (#3471)

Only modify the skolems to avoid leaking the sharper bounds to `quantified`.

The included test case was minimized from akka-camel
(src/main/scala/akka/camel/Consumer.scala).
Merge pull request #3533 from adriaanm/t8283

SI-8283 mutation-free bound inference for existentials