Silence warnings resulting from catch.hpp [blocks: #2310, #3425]

[tautschnig]

This is factored out from #3425 in an attempt to get at least this bit approved and merged.

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[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 606ffdf).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98117729
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[kroening]

Would there be merit in upstreaming these changes to catch.hpp?

[thk123]

This will need a manual TG bump - will create.

[tautschnig]

Would there be merit in upstreaming these changes to catch.hpp?

As in: fixing catch.hpp not to produce any warnings? Possibly so, but through the changes propose here we at least don't take any dependency on the proposals actually being picked up.

Half-related: I have a commit in the waiting to update catch.hpp to the most recent version, which will address the one local change we had made to catch.hpp earlier.

[tautschnig]

Thanks @thk123! Please just merge this PR when the corresponding TG change is ready.

[thk123]

@tautschnig done.