[FE] Collect inheritors of sealed classes from new places in computeSealedSubclasses

#KT-13495

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

@@ -0,0 +1,64 @@
+// ISSUE: KT-13495
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// !LANGUAGE: +FreedomForSealedClasses
+
+// FILE: Base.kt
+
+sealed class Base {
+    class A : Base()
+}
+
+// FILE: B.kt
+
+class B : Base()
+
+// FILE: Container.kt
+
+class Containter {
+    class C : Base()
+
+    inner class D : Base()
+}
+
+// FILE: main.kt
+
+fun test_OK(base: Base) {
+    val x = when (base) {
+        is Base.A -> 1
+        is B -> 2
+        is Containter.C -> 3
+        is Containter.D -> 4
+    }
+}
+
+fun test_error_1(base: Base) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (base) {
+        is B -> 2
+        is Containter.C -> 3
+        is Containter.D -> 4
+    }
+}
+
+fun test_error_2(base: Base) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (base) {
+        is Base.A -> 1
+        is Containter.C -> 3
+        is Containter.D -> 4
+    }
+}
+
+fun test_error_3(base: Base) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (base) {
+        is Base.A -> 1
+        is B -> 2
+        is Containter.D -> 4
+    }
+}
+
+fun test_error_4(base: Base) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (base) {
+        is Base.A -> 1
+        is B -> 2
+        is Containter.C -> 3
+    }
+}