[IR] support storing signatures for non-public symbols

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

@@ -48,6 +48,9 @@ abstract class Fir2IrBindableSymbol<out D : DeclarationDescriptor, B : IrDeclara
         return "Unbound public symbol for $signature"
     }
 
+    override val isPublicApi: Boolean
+        get() = true
+
     companion object {
         private const val GETTER_PREFIX = "<get"
         private const val SETTER_PREFIX = "<set"