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

#KT-20423

compiler/testData/diagnostics/tests/sealed/interfaces/sealedInterfacesDisabled.fir.kt

@@ -0,0 +1,5 @@
+// ISSUE: KT-20423
+// !LANGUAGE: -SealedInterfaces
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+sealed interface Base

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

@@ -0,0 +1,5 @@
+// ISSUE: KT-20423
+// !LANGUAGE: -SealedInterfaces
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+<!WRONG_MODIFIER_TARGET!>sealed<!> interface Base

compiler/testData/diagnostics/tests/sealed/interfaces/sealedInterfacesDisabled.txt

@@ -0,0 +1,7 @@
+package
+
+public sealed interface Base {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/sealed/interfaces/simpleSealedInterface.fir.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 = when (base) {
+        is A -> 1
+        is B.First -> 2
+        is B.Second -> 3
+        C.SomeValue -> 4
+        C.AnotherValue -> 5
+        D -> 6
+    }
+}

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
+    }
+}

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

@@ -0,0 +1,65 @@
+package
+
+public fun test_1(/*0*/ base: Base): kotlin.Unit
+public fun test_2(/*0*/ base: Base): kotlin.Unit
+
+public interface A : Base {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public sealed class B : Base {
+    internal constructor B()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public final class First : B {
+        public constructor First()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public final class Second : B {
+        public constructor Second()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+
+public sealed interface Base {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final enum class C : kotlin.Enum<C>, Base {
+    enum entry SomeValue
+
+    enum entry AnotherValue
+
+    private constructor C()
+    public final override /*1*/ /*fake_override*/ val name: kotlin.String
+    public final override /*1*/ /*fake_override*/ val ordinal: kotlin.Int
+    protected final override /*1*/ /*fake_override*/ fun clone(): kotlin.Any
+    public final override /*1*/ /*fake_override*/ fun compareTo(/*0*/ other: C): kotlin.Int
+    public final override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    protected/*protected and package*/ final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun finalize(): kotlin.Unit
+    public final override /*1*/ /*fake_override*/ /*isHiddenForResolutionEverywhereBesideSupercalls*/ fun getDeclaringClass(): java.lang.Class<C!>!
+    public final override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): C
+    public final /*synthesized*/ fun values(): kotlin.Array<C>
+}
+
+public object D : Base {
+    private constructor D()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}