[FIR] Make SyntheticPropertySymbol derived from FirAccessorSymbol

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/synthetic/FirSyntheticProperty.kt

@@ -13,9 +13,9 @@ import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.references.FirControlFlowGraphReference
 import org.jetbrains.kotlin.fir.references.impl.FirEmptyControlFlowGraphReference
+import org.jetbrains.kotlin.fir.symbols.impl.FirAccessorSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirBackingFieldSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirDelegateFieldSymbol
-import org.jetbrains.kotlin.fir.symbols.impl.FirPropertySymbol
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
@@ -27,7 +27,7 @@ class FirSyntheticProperty(
     override val returnTypeRef: FirTypeRef,
     override val name: Name,
     override val isVar: Boolean,
-    override val symbol: FirPropertySymbol,
+    override val symbol: FirAccessorSymbol,
     override val status: FirDeclarationStatus,
     override val resolvePhase: FirResolvePhase,
     override val getter: FirSyntheticPropertyAccessor,
@@ -40,7 +40,7 @@ class FirSyntheticProperty(
     constructor(
         session: FirSession,
         name: Name,
-        symbol: FirPropertySymbol,
+        symbol: FirAccessorSymbol,
         delegateGetter: FirSimpleFunction
     ) : this(
         session, delegateGetter.returnTypeRef, name, false, symbol,

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/impl/FirFunctionSymbol.kt

@@ -33,7 +33,7 @@ class FirConstructorSymbol(
     override val overriddenSymbol: FirConstructorSymbol? = null
 ) : FirFunctionSymbol<FirConstructor>(callableId)
 
-class FirAccessorSymbol(
+open class FirAccessorSymbol(
     callableId: CallableId,
     override val accessorId: CallableId
 ) : FirPropertySymbol(callableId), AccessorSymbol