core/trivial: replace "XXX" markers with "TODO"

"XXX" is used for tagging parts of code that still need work before
merging a patch. If you want to highlight/mark a comment which is merged
use either "TODO" or "FIXME".

Of course, even "TODO" and "FIXME" should be avoided in favor of just
doing/fixing it. Such things tend to never be done/fixed.
parent bc4c81fb
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