IR: .isBound API for F/O declarations

Fake override functions and properties may not be bound to symbols.
This API allows to know if such declaration is actually bound

compiler/ir/ir.tree.persistent/src/org/jetbrains/kotlin/ir/declarations/persistent/PersistentIrFunction.kt

@@ -75,7 +75,11 @@ internal class PersistentIrFakeOverrideFunction(
     private var _symbol: IrSimpleFunctionSymbol? = null
 
     override val symbol: IrSimpleFunctionSymbol
-        get() = _symbol ?: error("$this has not acquired a symbol yet")
+        get() = _symbol ?:
+            error("$this has not acquired a symbol yet")
+
+    override val isBound: Boolean
+        get() = _symbol != null
 
     @ObsoleteDescriptorBasedAPI
     override val descriptor

compiler/ir/ir.tree.persistent/src/org/jetbrains/kotlin/ir/declarations/persistent/PersistentIrProperty.kt

@@ -80,6 +80,9 @@ internal class PersistentIrFakeOverrideProperty(
     override val descriptor
         get() = _symbol?.descriptor ?: this.toIrBasedDescriptor()
 
+    override val isBound: Boolean
+        get() = _symbol != null
+
     @OptIn(ObsoleteDescriptorBasedAPI::class)
     override fun acquireSymbol(symbol: IrPropertySymbol): IrProperty {
         assert(_symbol == null) { "$this already has symbol _symbol" }