C front-end: printing enum type declarations with bitvector awareness

[tautschnig]

The constants are no longer stored as string literals, and instead use
the bitvector representation (currently: hexadecimal values).

• 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 #5611 (ecd5aff) into develop (e23bf5c) will increase coverage by 0.01%.
The diff coverage is 62.50%.

@@             Coverage Diff             @@
##           develop    #5611      +/-   ##
===========================================
+ Coverage    69.45%   69.47%   +0.01%     
===========================================
  Files         1243     1243              
  Lines       100614   100615       +1     
===========================================
+ Hits         69886    69905      +19     
+ Misses       30728    30710      -18     

Flag	Coverage Δ
cproversmt2	43.13% <12.50%> (-0.01%)	⬇️
regression	66.36% <62.50%> (+0.01%)	⬆️
unit	32.28% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	69.25% <62.50%> (-0.27%)	⬇️
src/goto-instrument/goto_program2code.cpp	67.91% <0.00%> (+0.38%)	⬆️
src/goto-instrument/dump_c.cpp	78.42% <0.00%> (+3.47%)	⬆️

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