Fix (part of) #6323: trust case class unapplies to produce checkable type tests

[abgruszecki]

If the type test comes from an unapply method of a case class, we trust it to generate a runtime-checkable type test, even if the test doesn't appear checkable. This fixes spurious warnings of the following kind:

sealed trait Foo[A, B]
final case class Bar[X](x: X) extends Foo[X, X]

def foo[A, B](value: Foo[A, B], a: A => Int): B = value match {
    case Bar(x) => a(x); x
}

-- Warning: test.scala:8:9 -----------------------------------------------------
8 |    case Bar() => a
  |         ^^^^^
  |         the type test for G.Bar[A] cannot be checked at runtime
one warning found

The cause of the warning, as can be gleamed from the error message, is that we infer Bar#X = A. Bar[A] is only guaranteed to be a subtype of Foo[A, B] under GADT constraint A = B, but by the time we check type tests, that information is gone.

Note that in the case where the unapply cannot possibly match, exhaustivity checker will still issue an error.

[odersky]

Otherwise LGTM