Replace use of deprecated nil_typet in c_bit_field_replacement_type [blocks: #3800]

[tautschnig]

Use optionalt as recommended in the deprecation note.

• 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.
• 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).
• 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]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 9cfcc9e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98611335
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[kroening]

Given that the return value isn't checked before use, this could also be a precondtion.

[tautschnig]

Given that the return value isn't checked before use, this could also be a precondtion.

Done.

That said, I am wondering whether we shouldn't either get rid of this code completely or make sure it actually is used everywhere (there are a lot more places handing bit-field types while not invoking this function).

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 83682da).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98716561