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

#KT-13495

compiler/testData/codegen/box/sealed/multipleFiles_enabled.kt

@@ -0,0 +1,24 @@
+// ISSUE: KT-13495
+// IGNORE_BACKEND_FIR: JVM_IR
+// !LANGUAGE: +FreedomForSealedClasses
+
+// FILE: a.kt
+
+sealed class Base {
+    class A : Base()
+}
+
+// FILE: b.kt
+
+class B : Base()
+
+// FILE: c.kt
+
+fun getLetter(base: Base): String = when (base) {
+    is Base.A -> "O"
+    is B -> "K"
+}
+
+fun box(): String {
+    return getLetter(Base.A()) + getLetter(B())
+}