SMT2-backend fix: constants

[kroening]

• Each commit message has a non-empty body, explaining why the change was made.
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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e4f3fc3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/92447983