Native: add KotlinThrowable.asError() to the generated Obj-C framework

This method can be useful when overriding a throwing Kotlin method in
Swift or Obj-C. In this case, a user can call asError to wrap Kotlin exception
to (NS)Error and then throw it to Kotlin, which will unwrap it back.

^KT-45127 Fixed

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -961,6 +961,10 @@ public abstract class KotlinBuiltIns {
         return isConstructedFromGivenClass(type, FqNames.throwable);
     }
 
+    public static boolean isThrowable(@NotNull ClassDescriptor descriptor) {
+        return classFqNameEquals(descriptor, FqNames.throwable.toUnsafe());
+    }
+
     public static boolean isKClass(@NotNull ClassDescriptor descriptor) {
         return classFqNameEquals(descriptor, FqNames.kClass);
     }