Fix types in use of set_polarity

[tautschnig]

MiniSat expects an lbool, not a bool. This remained unnoticed as the
method is unused and thus never instantiated. It only became apparent
when doing an explicit class template instantiation in the course of
other debugging work.

• 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 #6071 (0e34169) into develop (32e4a06) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6071      +/-   ##
===========================================
- Coverage    74.29%   74.25%   -0.05%     
===========================================
  Files         1445     1445              
  Lines       157477   157479       +2     
===========================================
- Hits        117005   116933      -72     
- Misses       40472    40546      +74     

Impacted Files	Coverage Δ
src/solvers/sat/satcheck_minisat2.cpp	72.34% <ø> (ø)
src/goto-symex/goto_state.cpp	84.48% <100.00%> (+0.55%)	⬆️
src/ansi-c/literals/convert_character_literal.cpp	51.21% <0.00%> (-17.08%)	⬇️
src/goto-programs/read_goto_binary.cpp	30.84% <0.00%> (-16.83%)	⬇️
src/ansi-c/literals/parse_float.cpp	81.73% <0.00%> (-14.43%)	⬇️
src/util/source_location.cpp	83.67% <0.00%> (-10.21%)	⬇️
src/util/file_util.cpp	71.42% <0.00%> (-5.36%)	⬇️
src/util/config.cpp	51.66% <0.00%> (-3.77%)	⬇️

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