Make ControlFlow #[must_use]

[LeSeulArtichaut]

Suggested by @oli-obk in #78182 (comment).
r? @scottmcm cc #75744

[scottmcm]

Hmm, I don't have a good sense for how to decide whether to do this. It makes total sense in the visitor recursion scenario, but seems suboptimal in the updated doctest's scenario of try_for_each with a closure that mutates locals. (And we saw in the fold_mut conversation (#76746) that using for_each with a mutable closure can be better than rewriting it to fold.)

Maybe @rust-lang/libs would have some advice? Sadly there's nothing I could find in the api-guidelines about #[must_use].

cc @NoraCodes and @ecstatic-morse, as other participants in the tracking issue, to comment on whether this makes sense in their situations.

[m-ou-se]

There's some discussion on #[must_use] in std here: #48926

[bors]

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