Work around Z3 not producing models for some quantified expressions

[tautschnig]

Until the Z3 bug-fix (see Z3Prover/z3#7743) is available widely work around the problem that Z3's preprocessing introduces by adding extra symbols. This may cause solving to take longer, to be determined empirically.

Fixes: #8679

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• 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.
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[codecov]

Codecov Report

❌ Patch coverage is 95.31250% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.39%. Comparing base (6b6c8d3) to head (e92bda3).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	95.31%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8703   +/-   ##
========================================
  Coverage    80.39%   80.39%           
========================================
  Files         1688     1688           
  Lines       207405   207415   +10     
  Branches        73       73           
========================================
+ Hits        166751   166760    +9     
- Misses       40654    40655    +1     

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