Fix #22051: only trust the type application part for case class unapplies

[noti0na1]

Although the current local class implementation is still unsound, this PR should bring back the warning.

Fix #22051

def boundary[T](body: (T => RuntimeException) => T): T =
  case class Break(value: T) extends RuntimeException
  try body(Break.apply)
  catch case Break(t) => t // error: pattern matching on local classes is unsound currently

def test =
  boundary[Int]: EInt =>
    val v: String = boundary[String]: EString =>
      throw EInt(3)
    v.length // a runtime error: java.lang.ClassCastException

-- [E092] Pattern Match Unchecked Warning: tests/neg/i22051.scala:6:13 ---------
6 |  catch case Break(t) => t // error: pattern matching on local classes is unsound currently
  |             ^
  |the type test for Break cannot be checked at runtime because it's a local class
  |
  | longer explanation available when compiling with `-explain`