Cleanup uses of std::move

[tautschnig]

Just remove std::move in case an unnamed object is returned, remove named
temporaries that add no value, and don't use const with std::move.

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[kroening]

I am sad about the interaction of const and the conversion to rvalue reference on return.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e08eb2f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95145326