[FIR] Fix exhaustiveness check in case of sealed subclass

The problem appear because check was performed only on direct children
of sealed class. To fix this, heritage tree is built and used in check

#KT-38989 Fixed

compiler/fir/analysis-tests/testData/resolve/exhaustiveness_sealedSubClass.kt

@@ -0,0 +1,54 @@
+sealed class A
+
+sealed class B : A()
+class C : A()
+
+sealed class D : B()
+sealed class E : B()
+
+fun test_1(e: A) {
+    val a = when (e) {
+        is C -> 1
+        is D -> 2
+        is E -> 3
+    }.plus(0)
+
+    val b = when (e) {
+        is B -> 1
+        is C -> 2
+    }.plus(0)
+
+    val c = when (e) {
+        is B -> 1
+        is C -> 2
+        is E -> 3
+        is D -> 4
+    }.plus(0)
+
+    val d = when (e) {
+        is E -> 1
+        is A -> 2
+    }.plus(0)
+}
+
+fun test_2(e: A) {
+    val a = when (e) {
+        is D -> 1
+        is E -> 2
+    }.<!UNRESOLVED_REFERENCE!>plus<!>(0)
+
+    val b = when (e) {
+        is B -> 1
+        is D -> 2
+        is E -> 3
+    }.<!UNRESOLVED_REFERENCE!>plus<!>(0)
+
+    val c = when (e) {
+        is B -> 1
+        is D -> 2
+    }.<!UNRESOLVED_REFERENCE!>plus<!>(0)
+
+    val d = when (e) {
+        is C -> 1
+    }.<!UNRESOLVED_REFERENCE!>plus<!>(0)
+}