[FE] Allow using sealed modifier on interface and compute sealed modality for them

#KT-20423

compiler/testData/diagnostics/tests/sealed/interfaces/simpleSealedInterface.kt

@@ -0,0 +1,38 @@
+// ISSUE: KT-20423
+// !LANGUAGE: +FreedomForSealedClasses +SealedInterfaces
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+sealed interface Base
+
+interface A : Base
+
+sealed class B : Base {
+    class First : B()
+    class Second : B()
+}
+
+enum class C : Base {
+    SomeValue, AnotherValue
+}
+
+object D : Base
+
+fun test_1(base: Base) {
+    val x = when (base) {
+        is A -> 1
+        is B -> 2
+        is C -> 3
+        is D -> 4
+    }
+}
+
+fun test_2(base: Base) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (base) {
+        is A -> 1
+        is B.First -> 2
+        is B.Second -> 3
+        C.SomeValue -> 4
+        C.AnotherValue -> 5
+        D -> 6
+    }
+}