Port IdeHighlightIndex to ArcBox

Initial work on #647 (closed). Started with this struct because it was the easiest to trace and make sure it still works after the change. Had to do some reading on the g_atomic_rc_box_* API, so let me know if there is anything wrong in the code.

If all looks good, I'll continue with the rest (most of these structs are in libide/diagnostics/).

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