Add return type to functions in global namespace

Working on it.

Pushed my work so far using the rector rules in the IS to MR !11501. There are 67 functions remaining, using the steps to reproduce.

Pushed some manual updates, this is down to 39 now. Definitely leaning towards getting void returns in first then splitting the rest.

I agree, voids are the easiest to review.

Now that the low-hanging fruit is committed I've rebased the MR and commented on some of the less obvious changes. I've updated the issue summary with a proposed resolution on how to split the remaining work. Setting NR for agreement on that approach, and for eyes on the MR.