[JS IR BE] Fix fake override functions resolve

Use signature of function 'target' (a real function that will
be called instad of fake override) to make boxing/unboxing decision for
return value and type parameters on call site.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -360,3 +360,5 @@ fun Scope.createTemporaryVariableWithWrappedDescriptor(
         irExpression, nameHint, isMutable, origin, descriptor
     ).apply { descriptor.bind(this) }
 }
+
+val IrFunction.isOverridable: Boolean get() = this is IrSimpleFunction && this.isOverridable