signed left-shift overflow depends on standard version

[kroening]

The semantics of signed left shifts are contentious for the case
that a '1' is shifted into the signed bit.
Assuming 32-bit integers, 1<<31 is implementation-defined
in ANSI C and C++98, but is explicitly undefined by C99,
C11 and C++11.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 3e0164e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96381233

[smowton]

Add an extra variant of the test with --c99?

[kroening]

Sure, done

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 689b2da).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96410366

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ae78609).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96465454

[smowton]

@kroening it looks like you renamed the test.desc to -c89.desc but never added the -c99.desc version which I assume existed on your local system