Cleanup expr -> XML conversion of c_bool constants

[tautschnig]

This is largely the same as handling any other signed/unsignedbv, thus make them
share code.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0827858).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99314424