[IR] Mark IrExpressionBodyImpl with an opt-in annotation

KT-59318

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrBuilderWithPluginContext.kt

@@ -180,7 +180,7 @@ interface IrBuilderWithPluginContext {
                 field.startOffset,
                 field.endOffset
             )
-            field.initializer = IrExpressionBodyImpl(builder.initializer())
+            field.initializer = factory.createExpressionBody(builder.initializer())
             field.annotations += createAnnotationCallWithoutArgs(compilerContext.jvmFieldClassSymbol)
         }
     }