[IR] add a TODO for the privateSignature

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/symbols/IrSymbol.kt

@@ -37,6 +37,7 @@ interface IrSymbol {
 
     val signature: IdSignature?
 
+    // TODO: remove once JS IR IC migrates to a different stable tag generation scheme
     // Used to store signatures in private symbols for JS IC
     var privateSignature: IdSignature?
 }