In short: You can't.
In long:
IonCube is an encoder, and Xdebug is a debugger - not a decoder. It does not know how to read encoded files and even if it could get into the process of intercepting the PHP things that it does, it still won't be able to show you the source code.
When a file is encoded, it is first converted from text into "opcodes"—that is an internal representation of binary stuff which PHP can execute. At this stage, it is already useless for Xdebug.
And then it is encoded, which makes it even less possible to do anything with.