[FIR] Make SyntheticPropertySymbol derived from FirAccessorSymbol

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassEnhancementScope.kt

@@ -106,12 +106,12 @@ class JavaClassEnhancementScope(
                 return symbol
             }
             is FirSyntheticProperty -> {
-                original as FirAccessorSymbol
+                val accessorSymbol = firElement.symbol
                 val enhancedFunctionSymbol = enhanceMethod(
-                    firElement.getter.delegate, original.accessorId, original.accessorId.callableName
+                    firElement.getter.delegate, accessorSymbol.accessorId, accessorSymbol.accessorId.callableName
                 )
                 val enhancedProperty = FirSyntheticProperty(
-                    session, name, FirAccessorSymbol(original.callableId, original.accessorId),
+                    session, name, FirAccessorSymbol(accessorSymbol.callableId, accessorSymbol.accessorId),
                     enhancedFunctionSymbol.fir as FirSimpleFunction
                 )
                 return enhancedProperty.symbol

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassUseSiteMemberScope.kt

@@ -49,7 +49,7 @@ class JavaClassUseSiteMemberScope(
         syntheticPropertyName: Name,
         overrideCandidates: MutableSet<FirCallableSymbol<*>>,
         isGetter: Boolean
-    ): FirPropertySymbol? {
+    ): FirAccessorSymbol? {
         if (functionSymbol !is FirNamedFunctionSymbol) {
             return null
         }