If you accidentally modify the system library files, do not panic, look at the following solutions.
first: Of course it is used "command + Z"
If not, look below：
1、enter preferences(command + ,) , click location button
2、select first row(Default Data), and then click Right arrow enter the path，you can look at the DerivedData file , Select all the files inside DerivedData and delete.
3、last: clean and build your project.....
If not, reinstall Xcode.