Drop redundant fields around AbstractFir2IrLazyFunction

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

@@ -59,7 +59,9 @@ abstract class AbstractFir2IrLazyFunction<F : FirMemberDeclaration>(
     override val isExpect: Boolean
         get() = fir.isExpect
 
-    override var body: IrBody? = null
+    override var body: IrBody?
+        get() = null
+        set(_) = error("We should never need to store body of external functions.")
 
     override var visibility: DescriptorVisibility by lazyVar {
         components.visibilityConverter.convertToDescriptorVisibility(fir.visibility)

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazyPropertyAccessor.kt

@@ -18,15 +18,15 @@ import org.jetbrains.kotlin.serialization.deserialization.descriptors.Deserializ
 
 class Fir2IrLazyPropertyAccessor(
     components: Fir2IrComponents,
-    override val startOffset: Int,
-    override val endOffset: Int,
-    override var origin: IrDeclarationOrigin,
+    startOffset: Int,
+    endOffset: Int,
+    origin: IrDeclarationOrigin,
     private val firAccessor: FirPropertyAccessor?,
     private val isSetter: Boolean,
     private val firParentProperty: FirProperty,
     firParentClass: FirRegularClass,
-    override val symbol: Fir2IrSimpleFunctionSymbol,
-    override val isFakeOverride: Boolean
+    symbol: Fir2IrSimpleFunctionSymbol,
+    isFakeOverride: Boolean
 ) : AbstractFir2IrLazyFunction<FirMemberDeclaration>(components, startOffset, endOffset, origin, symbol, isFakeOverride) {
     init {
         symbol.bind(this)

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazySimpleFunction.kt

@@ -18,13 +18,13 @@ import org.jetbrains.kotlin.serialization.deserialization.descriptors.Deserializ
 
 class Fir2IrLazySimpleFunction(
     components: Fir2IrComponents,
-    override val startOffset: Int,
-    override val endOffset: Int,
-    override var origin: IrDeclarationOrigin,
+    startOffset: Int,
+    endOffset: Int,
+    origin: IrDeclarationOrigin,
     override val fir: FirSimpleFunction,
     firParent: FirRegularClass,
-    override val symbol: Fir2IrSimpleFunctionSymbol,
-    override val isFakeOverride: Boolean
+    symbol: Fir2IrSimpleFunctionSymbol,
+    isFakeOverride: Boolean
 ) : AbstractFir2IrLazyFunction<FirSimpleFunction>(components, startOffset, endOffset, origin, symbol, isFakeOverride) {
     init {
         symbol.bind(this)