[FE] Allow declare sealed class inheritors in different files in one module

#KT-13495

compiler/testData/diagnostics/tests/sealed/MultipleFiles_enabled.fir.kt

@@ -0,0 +1,30 @@
+// ISSUE: KT-13495
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// !LANGUAGE: +FreedomForSealedClasses
+
+// FILE: a.kt
+
+sealed class Base {
+    class A : Base()
+}
+
+// FILE: b.kt
+
+class B : <!HIDDEN!>Base<!>()
+
+// FILE: c.kt
+
+class Container {
+    class C : <!HIDDEN, SEALED_SUPERTYPE!>Base<!>()
+
+    inner class D : <!HIDDEN, SEALED_SUPERTYPE!>Base<!>()
+}
+
+// FILE: d.kt
+
+fun test(base: Base) {
+    val x = when (base) {
+        is Base.A -> 1
+        is B -> 2
+    }
+}

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

@@ -0,0 +1,30 @@
+// ISSUE: KT-13495
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// !LANGUAGE: +FreedomForSealedClasses
+
+// FILE: a.kt
+
+sealed class Base {
+    class A : Base()
+}
+
+// FILE: b.kt
+
+class B : Base()
+
+// FILE: c.kt
+
+class Container {
+    class C : <!SEALED_SUPERTYPE!>Base<!>()
+
+    inner class D : <!SEALED_SUPERTYPE!>Base<!>()
+}
+
+// FILE: d.kt
+
+fun test(base: Base) {
+    val x = when (base) {
+        is Base.A -> 1
+        is B -> 2
+    }
+}

compiler/testData/diagnostics/tests/sealed/MultipleFiles_enabled.txt

@@ -0,0 +1,45 @@
+package
+
+public fun test(/*0*/ base: Base): kotlin.Unit
+
+public final class B : Base {
+    public 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 sealed class Base {
+    internal constructor 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 class A : Base {
+        public constructor A()
+        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 Container {
+    public constructor Container()
+    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 C : Base {
+        public constructor C()
+        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 inner class D : Base {
+        public 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
+    }
+}