format_expr now outputs constants of type c_bit_field

[kroening]

This adds a case for outputting constants that have c_bit_field type as
numbers.

• 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.

[codecov]

Codecov Report

Merging #6280 (645c07b) into develop (4844d22) will decrease coverage by 0.26%.
The diff coverage is 97.56%.

@@             Coverage Diff             @@
##           develop    #6280      +/-   ##
===========================================
- Coverage    76.17%   75.90%   -0.27%     
===========================================
  Files         1484     1492       +8     
  Lines       162317   162714     +397     
===========================================
- Hits        123646   123511     -135     
- Misses       38671    39203     +532     

Impacted Files	Coverage Δ
jbmc/src/jdiff/jdiff_parse_options.h	100.00% <ø> (ø)
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/goto-analyzer/goto_analyzer_parse_options.cpp	71.98% <ø> (-0.34%)	⬇️
src/goto-cc/goto_cc_languages.cpp	100.00% <ø> (ø)
src/goto-diff/goto_diff_parse_options.h	100.00% <ø> (ø)
src/goto-harness/goto_harness_parse_options.cpp	68.37% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
...rc/memory-analyzer/memory_analyzer_parse_options.h	100.00% <ø> (ø)
src/solvers/smt2_incremental/smt_sorts.cpp	94.11% <ø> (ø)
src/solvers/smt2_incremental/smt_sorts.h	100.00% <ø> (ø)
... and 54 more

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