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/src/org/jetbrains/kotlin/ir/declarations/IrFakeOverrideFunction.kt

@@ -14,4 +14,6 @@ interface IrFakeOverrideFunction : IrDeclaration {
     var modality: Modality
 
     fun acquireSymbol(symbol: IrSimpleFunctionSymbol): IrSimpleFunction
+
+    val isBound: Boolean
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrFakeOverrideProperty.kt

@@ -16,4 +16,6 @@ interface IrFakeOverrideProperty : IrDeclaration {
     var setter: IrSimpleFunction?
 
     fun acquireSymbol(symbol: IrPropertySymbol): IrProperty
+
+    val isBound: Boolean
 }