A key combination inserts invisible and festy character in Xcode
I don't think this is possible in Xcode.

This happens because ctrl+x indicates to Xcode that you want to type one of the ASCII control characters. The next thing you type tells it which one. Pressing ctrl+s inserts an ASCII 19, "device control 3". Different letters insert different control characters-- see the chart at that link for a full list. Control characters are not printable, so it's invisible. Unfortunately Xcode's option to show invisibles (Edit menu --> Invisibles) doesn't show ASCII control characters, so you end up with something Xcode doesn't like but that you can't see.

You might be able to use Karabiner to deal with this, I'm not sure though.