[JS IR] store signatures for private symbols separately from public

Those signatures are used in JS IR IC

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/symbols/Fir2IrBindableSymbol.kt

@@ -43,6 +43,8 @@ abstract class Fir2IrBindableSymbol<out D : DeclarationDescriptor, B : IrDeclara
     override val hasDescriptor: Boolean
         get() = false
 
+    override var privateSignature: IdSignature? = null
+
     override fun toString(): String {
         if (isBound) return owner.render()
         return "Unbound public symbol for $signature"