[FIR] Add some safe API to synthetic property symbols

Previously there were no way to get delegated function symbol of accessor
  symbol of synthetic property

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

@@ -15,8 +15,8 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirPropertyAccessorImpl
 import org.jetbrains.kotlin.fir.expressions.FirAnnotation
 import org.jetbrains.kotlin.fir.expressions.FirBlock
 import org.jetbrains.kotlin.fir.references.FirControlFlowGraphReference
-import org.jetbrains.kotlin.fir.symbols.impl.FirPropertyAccessorSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirPropertySymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirSyntheticPropertyAccessorSymbol
 import org.jetbrains.kotlin.fir.types.ConeSimpleKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
@@ -70,7 +70,7 @@ class FirSyntheticPropertyAccessor @FirImplementationDetail internal constructor
     override val attributes: FirDeclarationAttributes
         get() = delegate.attributes
 
-    override val symbol: FirPropertyAccessorSymbol = FirPropertyAccessorSymbol().apply {
+    override val symbol: FirSyntheticPropertyAccessorSymbol = FirSyntheticPropertyAccessorSymbol().apply {
         bind(this@FirSyntheticPropertyAccessor)
     }
 

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

@@ -9,9 +9,11 @@ import org.jetbrains.kotlin.fir.FirLabel
 import org.jetbrains.kotlin.fir.contracts.FirResolvedContractDescription
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.synthetic.FirSyntheticProperty
+import org.jetbrains.kotlin.fir.declarations.synthetic.FirSyntheticPropertyAccessor
 import org.jetbrains.kotlin.fir.expressions.FirDelegatedConstructorCall
 import org.jetbrains.kotlin.fir.references.FirControlFlowGraphReference
 import org.jetbrains.kotlin.fir.references.toResolvedConstructorSymbol
+import org.jetbrains.kotlin.fir.symbols.SymbolInternals
 import org.jetbrains.kotlin.fir.symbols.lazyResolveToPhase
 import org.jetbrains.kotlin.mpp.ConstructorSymbolMarker
 import org.jetbrains.kotlin.mpp.FunctionSymbolMarker
@@ -83,8 +85,15 @@ class FirConstructorSymbol(callableId: CallableId) : FirFunctionSymbol<FirConstr
 abstract class FirSyntheticPropertySymbol(propertyId: CallableId, val getterId: CallableId) : FirPropertySymbol(propertyId) {
     abstract fun copy(): FirSyntheticPropertySymbol
 
+    @SymbolInternals
     val syntheticProperty: FirSyntheticProperty
         get() = fir as FirSyntheticProperty
+
+    override val getterSymbol: FirSyntheticPropertyAccessorSymbol?
+        get() = super.getterSymbol as FirSyntheticPropertyAccessorSymbol?
+
+    override val setterSymbol: FirSyntheticPropertyAccessorSymbol?
+        get() = super.setterSymbol as FirSyntheticPropertyAccessorSymbol?
 }
 
 // ------------------------ unnamed ------------------------
@@ -95,10 +104,18 @@ class FirAnonymousFunctionSymbol : FirFunctionWithoutNameSymbol<FirAnonymousFunc
     val label: FirLabel? get() = fir.label
 }
 
-class FirPropertyAccessorSymbol : FirFunctionWithoutNameSymbol<FirPropertyAccessor>(Name.identifier("accessor")) {
+open class FirPropertyAccessorSymbol : FirFunctionWithoutNameSymbol<FirPropertyAccessor>(Name.identifier("accessor")) {
     val isGetter: Boolean get() = fir.isGetter
     val isSetter: Boolean get() = fir.isSetter
-    val propertySymbol get() = fir.propertySymbol
+    open val propertySymbol: FirPropertySymbol get() = fir.propertySymbol
+}
+
+class FirSyntheticPropertyAccessorSymbol : FirPropertyAccessorSymbol() {
+    override val propertySymbol: FirSyntheticPropertySymbol
+        get() = super.propertySymbol as FirSyntheticPropertySymbol
+
+    val delegateFunctionSymbol: FirNamedFunctionSymbol
+        get() = (fir as FirSyntheticPropertyAccessor).delegate.symbol
 }
 
 class FirErrorFunctionSymbol : FirFunctionWithoutNameSymbol<FirErrorFunction>(Name.identifier("error"))

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

@@ -26,10 +26,10 @@ open class FirPropertySymbol(callableId: CallableId, ) : FirVariableSymbol<FirPr
     val isLocal: Boolean
         get() = fir.isLocal
 
-    val getterSymbol: FirPropertyAccessorSymbol?
+    open val getterSymbol: FirPropertyAccessorSymbol?
         get() = fir.getter?.symbol
 
-    val setterSymbol: FirPropertyAccessorSymbol?
+    open val setterSymbol: FirPropertyAccessorSymbol?
         get() = fir.setter?.symbol
 
     val backingFieldSymbol: FirBackingFieldSymbol?