Fix treatment of bottom types in OrType#join and baseType computations

[odersky]

Fixes #11968

[noti0na1]

Could you extend test to this? Thanks

// Test selecting member of nullable classes

class A {
  def get(): Int = 0
}

class B extends A {}

class C extends A {}

def test1 = {
  val s: String | Null = ???
  val l = s.length

  val a: A | Null = new A
  a.get()

  val bc: B | C = new B
  bc.get()

  val bcn: B | (C | Null) = new C
  bcn.get()

  val bnc: (B | Null) | C = null
  bnc.get()

  val abcn: A | B | C | Null = new A
  abcn.get()
}

def test2 = {
  val s: String | Nothing = ???
  val l = s.length

  val a: A | Nothing = new A
  a.get()

  val bc: B | C = new B
  bc.get()

  val bcn: B | (C | Nothing) = new C
  bcn.get()

  val bnc: (B | Nothing) | C = new B
  bnc.get()

  val abcn: A | B | C | Nothing = new A
  abcn.get()
}

It seems subtyping check on both side is not enough.

-- [E008] Not Found Error: Stest.scala:22:6 ------------------------------------
22 |  bcn.get()
   |  ^^^^^^^
   |  value get is not a member of B | (C | Null)
-- [E008] Not Found Error: Stest.scala:25:6 ------------------------------------
25 |  bnc.get()
   |  ^^^^^^^
   |  value get is not a member of B | Null | C
-- [E008] Not Found Error: Stest.scala:42:6 ------------------------------------
42 |  bcn.get()
   |  ^^^^^^^
   |  value get is not a member of B | (C | Nothing)
-- [E008] Not Found Error: Stest.scala:45:6 ------------------------------------
45 |  bnc.get()
   |  ^^^^^^^
   |  value get is not a member of B | Nothing | C

[noti0na1]

LGTM