JVM IR: support -Xmultifile-parts-inherit mode

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrClass.kt

@@ -32,7 +32,7 @@ interface IrClass :
     override var visibility: Visibility
 
     val kind: ClassKind
-    val modality: Modality
+    var modality: Modality
     val isCompanion: Boolean
     val isInner: Boolean
     val isData: Boolean

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrClassImpl.kt

@@ -38,7 +38,7 @@ class IrClassImpl(
     override val name: Name,
     override val kind: ClassKind,
     override var visibility: Visibility,
-    override val modality: Modality,
+    override var modality: Modality,
     override val isCompanion: Boolean,
     override val isInner: Boolean,
     override val isData: Boolean,

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/lazy/IrLazyClass.kt

@@ -28,7 +28,7 @@ class IrLazyClass(
     override val name: Name,
     override val kind: ClassKind,
     override var visibility: Visibility,
-    override val modality: Modality,
+    override var modality: Modality,
     override val isCompanion: Boolean,
     override val isInner: Boolean,
     override val isData: Boolean,