IR: make isExternal immutable

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/AbstractFir2IrLazyFunction.kt

@@ -58,9 +58,8 @@ abstract class AbstractFir2IrLazyFunction<F : FirMemberDeclaration>(
     override val isInline: Boolean
         get() = fir.isInline
 
-    override var isExternal: Boolean by lazyVar {
-        fir.isExternal
-    }
+    override val isExternal: Boolean
+        get() = fir.isExternal
 
     override val isExpect: Boolean
         get() = fir.isExpect

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazyClass.kt

@@ -84,11 +84,8 @@ class Fir2IrLazyClass(
     override val isData: Boolean
         get() = fir.isData
 
-    override var isExternal: Boolean
+    override val isExternal: Boolean
         get() = fir.isExternal
-        set(_) {
-            error("Mutating Fir2Ir lazy elements is not possible")
-        }
 
     override val isInline: Boolean
         get() = fir.isInline

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazyConstructor.kt

@@ -50,11 +50,8 @@ class Fir2IrLazyConstructor(
     override val isInline: Boolean
         get() = fir.isInline
 
-    override var isExternal: Boolean
+    override val isExternal: Boolean
         get() = fir.isExternal
-        set(_) {
-            error("Mutating Fir2Ir lazy elements is not possible")
-        }
 
     override val isExpect: Boolean
         get() = fir.isExpect

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazyProperty.kt

@@ -63,11 +63,8 @@ class Fir2IrLazyProperty(
     override val isDelegated: Boolean
         get() = fir.delegate != null
 
-    override var isExternal: Boolean
+    override val isExternal: Boolean
         get() = fir.isExternal
-        set(_) {
-            error("Mutating Fir2Ir lazy elements is not possible")
-        }
 
     override val isExpect: Boolean
         get() = fir.isExpect