Mark constant_exprt::value_is_zero_string protected [depends-on: 8675]

[tautschnig]

The value representation is an implementation detail. Callers should use methods at a higher level of abstraction.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.46%. Comparing base (06665e7) to head (8cc6337).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8455      +/-   ##
===========================================
- Coverage    80.46%   80.46%   -0.01%     
===========================================
  Files         1698     1698              
  Lines       208510   208508       -2     
  Branches        73       73              
===========================================
- Hits        167784   167776       -8     
- Misses       40726    40732       +6     

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