Add config for gccdiag

I've written a tool that helps you get lint information from `gcc`, but with regards to the `compile_commands.json`. It's better than just calling `gcc -o /dev/null` because it gets the proper flags from the compilation database. This allows the compilation (which does linting alongside) to succeed almost always.

The tool can be found [here](https://gitlab.com/andrejr/gccdiag). I have a diagnostic-langserver config for it in the readme there, take a look.

Would you be interested in adding the config to the wiki here?

Start with the diagnostic-langserver configuration in the linked gccdiag README, then review the project's wiki conventions. Add the gccdiag configuration to the appropriate wiki page and verify that it is represented accurately.

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