Ensure result of to_predicates is deterministic

[tautschnig]

The order of entries in an unordered map would previously yield varying
results. This caused spurious unit test failures as unit tests perform
exact string matching on the output.

See https://github.com/diffblue/cbmc/pull/5740/checks?check_run_id=3321470809 for an example of a spuriously failing run.

• 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.

[tautschnig]

@jezhiggins You might care to take a look.

[martin-cs]

Sorry I missed this and thanks for the fix.

[jezhiggins]

Thanks!

[codecov]

Codecov Report

Merging #6296 (8065a69) into develop (abced8c) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff            @@
##           develop    #6296   +/-   ##
========================================
  Coverage    75.96%   75.97%           
========================================
  Files         1508     1508           
  Lines       163292   163380   +88     
========================================
+ Hits        124052   124131   +79     
- Misses       39240    39249    +9     

Impacted Files	Coverage Δ
src/solvers/flattening/bv_pointers.cpp	82.56% <66.66%> (-0.11%)	⬇️
...yses/variable-sensitivity/abstract_environment.cpp	94.47% <100.00%> (+0.01%)	⬆️
...-sensitivity/abstract_environment/to_predicate.cpp	100.00% <100.00%> (ø)
...nsitivity/three_way_merge_abstract_interpreter.cpp	93.02% <0.00%> (-1.32%)	⬇️
src/analyses/ai.h	57.36% <0.00%> (-0.78%)	⬇️
src/goto-analyzer/build_analyzer.cpp	94.91% <0.00%> (ø)
...sensitivity/three_way_merge_abstract_interpreter.h	100.00% <0.00%> (ø)
...ensitivity/variable_sensitivity_dependence_graph.h	79.31% <0.00%> (ø)
...sitivity/variable_sensitivity_dependence_graph.cpp	84.58% <0.00%> (ø)
src/util/sharing_map.h	88.14% <0.00%> (+0.03%)	⬆️
... and 1 more

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