Constify ControlFlow methods without unstable bounds

[nxsaken]

This PR constifies some of the methods on ControlFlow. There's no tracking issue for this.

#148285 constifies the rest of the methods involving const_trait_impl.

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[workingjubilee]

Note that some of the constifications touch #143874 -- let me know if I should split off the ones that don't.

Yes.

[nxsaken]

Moved the const_trait_impl related const–ns to a separate PR: #148285. Should I give them separate feature flags and create tracking issues for each?