[FIR] Make anonymous initializer lazy in RawFirBuilder

Fourth step for KT-52615

Merge-request: KT-MR-7798
Merged-by: Egor Kulikov <Egor.Kulikov@jetbrains.com>

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -437,7 +437,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
         }
 
         anonymousInitializer.configure {
-            +body(nullable = true)
+            +body(nullable = true, withReplace = true)
             +symbol("FirAnonymousInitializerSymbol")
         }