SMT-LIB2 now outputs negative integer/rational literals correctly

[kroening]

SMT-LIB2 does not have negative integer/rational literals, and it's
necessary to use a unary minus.

• 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 #6301 (45222f0) into develop (63203b1) will decrease coverage by 0.00%.
The diff coverage is 20.00%.

@@             Coverage Diff             @@
##           develop    #6301      +/-   ##
===========================================
- Coverage    75.98%   75.98%   -0.01%     
===========================================
  Files         1510     1510              
  Lines       163465   163473       +8     
===========================================
  Hits        124208   124208              
- Misses       39257    39265       +8     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.cpp	66.50% <0.00%> (-0.20%)	⬇️
src/util/floatbv_expr.cpp	0.00% <0.00%> (ø)
src/util/floatbv_expr.h	73.58% <ø> (ø)
src/util/ieee_float.cpp	88.01% <0.00%> (ø)
src/util/ieee_float.h	85.71% <ø> (ø)
...yses/variable-sensitivity/abstract_environment.cpp	95.02% <100.00%> (ø)
...-sensitivity/abstract_environment/to_predicate.cpp	100.00% <100.00%> (ø)

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