Forbid enum extending enum

[Blaisorblade]

We should forbid this code (discovered in #4961), we already forbid case classes/objects extending enum:

object enumTest {
  enum Test {
    case A
    case B
    case C
  }

  enum Test2 extends Test {
    case D
  }
}

[Blaisorblade]

I'm guessing refchecks because we can't necessarily resolve Test in the extends clause before typer (beware cycles!), but another phase (typer/posttyper) might be appropriate.