This was dropped in scala#18201 which restricted SAM types to valid parent types,
but it turns out that there is code in the wild that relies on refinements
being allowed here.

To support this properly, we had to enhance ExpandSAMs to move refinements into
type members to pass Ycheck (previous Scala 3 releases would accept the code in
tests/run/i18315.scala but fail Ycheck).

Fixes scala#18315.

Fixes scala#20080