Exhaustive when support for sealed classes #KT-7606 Fixed

When on sealed can use is (both for derived classes and objects) or just comparison (only for derived objects).
A pack of tests provided.

compiler/testData/diagnostics/tests/sealed/NonExhaustiveWhenWithAdditionalCase.kt

@@ -0,0 +1,22 @@
+sealed class Sealed(val x: Int) {
+    interface ITuple {
+        val x: Int
+        val y: Int
+    }
+    class Tuple(override val x: Int, override val y: Int): ITuple
+    object First: Sealed(12)
+    open class NonFirst(tuple: Tuple): Sealed(tuple.x), ITuple {
+        override val y: Int = tuple.y
+        object Second: NonFirst(Tuple(34, 2))
+        class Third: NonFirst(Tuple(56, 3))
+    }
+}
+
+fun foo(s: Sealed): Int {
+    return <!NO_ELSE_IN_WHEN!>when<!>(s) {
+        is Sealed.First -> 1
+        !is <!INCOMPATIBLE_TYPES!>Sealed.ITuple<!> -> 0
+        // else required
+    }
+}
+