IR: make isExternal immutable

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

@@ -36,7 +36,7 @@ class IrClassImpl(
     override val isCompanion: Boolean = false,
     override val isInner: Boolean = false,
     override val isData: Boolean = false,
-    override var isExternal: Boolean = false,
+    override val isExternal: Boolean = false,
     override val isInline: Boolean = false,
     override val isExpect: Boolean = false,
     override val isFun: Boolean = false,

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

@@ -38,7 +38,7 @@ class IrConstructorImpl(
     override var visibility: DescriptorVisibility,
     returnType: IrType,
     override val isInline: Boolean,
-    override var isExternal: Boolean,
+    override val isExternal: Boolean,
     override val isPrimary: Boolean,
     override val isExpect: Boolean,
     override val containerSource: DeserializedContainerSource? = null,

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

@@ -36,7 +36,7 @@ class IrFieldImpl(
     override var type: IrType,
     override var visibility: DescriptorVisibility,
     override val isFinal: Boolean,
-    override var isExternal: Boolean,
+    override val isExternal: Boolean,
     override val isStatic: Boolean,
 ) : IrField() {
     init {

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

@@ -29,7 +29,7 @@ abstract class IrFunctionCommonImpl(
     override var visibility: DescriptorVisibility,
     returnType: IrType,
     override val isInline: Boolean,
-    override var isExternal: Boolean,
+    override val isExternal: Boolean,
     override val isTailrec: Boolean,
     override val isSuspend: Boolean,
     override val isOperator: Boolean,

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

@@ -37,7 +37,7 @@ abstract class IrPropertyCommonImpl(
     override val isConst: Boolean,
     override val isLateinit: Boolean,
     override val isDelegated: Boolean,
-    override var isExternal: Boolean,
+    override val isExternal: Boolean,
     override val isExpect: Boolean,
     override val containerSource: DeserializedContainerSource?,
 ) : IrProperty() {