Clickable code navigation

Description

Code, most of the time, references classes, methods and filenames that are not in the current file.

When viewing the code on GitLab, it would be great to be able to click on them and see the definition on the file that contains them.

Source: https://news.ycombinator.com/item?id=13192317

Example: http://lxr.linux.no/linux+v4.9/kernel/async.c