forward port knowledge of private and sealed traits in exhaustivity warnings

[bishabosha]

port fixes from scala 2 that closed scala/scala#9211

Compiler version

3.1.1

Minimized code

trait A {
  sealed abstract class X
  private class X1 extends X with X2 { }
  private trait X2 extends X
  sealed trait X3 extends X

  def f(x: X) = x match {
    case _: X1 => 0
  }
}

Output

-- [E029] Pattern Match Exhaustivity Warning: ----------------------------------
7 |  def f(x: X) = x match {
  |                ^
  |                match may not be exhaustive.
  |
  |                It would fail on pattern case: _: X2, _: X3

Expectation

No warning, like scala 2