Fix handling of case range statement in special case

[NlightNFotis]

In the case where a gcc case range switch statement has a case that reduces to an empty expression, both gcc and clang reluctantly accept it, producing warnings. CBMC up until now was asserting against that case, and this divergence in behaviour was unwanted.

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[tautschnig]

Couldn't we have just the new commit in this PR? I think that's an independent problem and we shouldn't entangle that with the other PR. In fact I think #2905 should eventually depend on this one.

Edit: sorry, I should have read the comment on #2905 first. #2905 is merged, this one should now be rebased (and then likely needs more discussion).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: a076249).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86181339

[NlightNFotis]

@tautschnig I have rebased it appropriately now. It should be ready for more discussion/review now.

[tautschnig]

Could you please add another test case where all/the only case is an empty one? Basically a variant of the existing goto_convert_switch_range_empty test without the default case.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 7685476).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86275983

[NlightNFotis]

@tautschnig Yes, done.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 5da7d4f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86295457