Unreachability check for pattern is not precise enough when the scrutinee is a union

[smarter]

class A
class B
class C

object Test {
  def foo(x: A) = x match {
    case x: B => x // as expected, error: this case is unreachable since class A is not a subclass of class B
    case _ =>
  }

  def bar(x: A | B) = x match {
    case x: C => x // no error!
    case _ =>
  }
}

The logic handling this is in TypeTestCast#interceptTypeApply#interceptWith#transformIsInstanceOf which computes a class symbol for the scrutinee:

      def foundCls = effectiveClass(expr.tpe.widen)

So when the pattern has type A | B, testCls will be Any. This logic needs to be adapted to handle union types where there's more than one possible class symbol.