Support sealed class inheritors in the same file

#KT-11573 Fixed

compiler/testData/diagnostics/tests/when/TopLevelSealed.kt

@@ -0,0 +1,22 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+sealed class A {
+    class B: A() {
+        class C: A()
+    }
+}
+
+class D: A()
+
+fun test(a: A) {
+    val nonExhaustive = <!NO_ELSE_IN_WHEN!>when<!> (a) {
+        is A.B -> "B"
+        is A.B.C -> "C"
+    }
+
+    val exhaustive = when (a) {
+        is A.B -> "B"
+        is A.B.C -> "C"
+        is D -> "D"
+    }
+}