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

#KT-13495

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

@@ -0,0 +1,39 @@
+// ISSUE: KT-13495
+// !LANGUAGE: +FreedomForSealedClasses
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+// FILE: base.kt
+
+package foo
+
+class Container {
+    sealed class Base
+}
+
+// FILE: a.kt
+
+package foo
+
+class A : Container.Base()
+
+// FILE: b.kt
+
+package foo
+
+class BContainer {
+    class B : Container.Base()
+
+    inner class C : Container.Base()
+}
+
+// FILE: test.kt
+
+package foo
+
+fun test(base: Container.Base) {
+    val x = when (base) {
+        is A -> 1
+        is BContainer.B -> 2
+        is BContainer.C -> 3
+    }
+}