Do not clear the clause set between calls to the external SAT solver

[zhassan-aws]

Revert the change done in #7346. Deleting all clauses between calls to the external SAT solver results in losing all the clauses for the equation. The generated CNF for subsequent calls thus ends up including the clauses for the remaining properties/goals only.

Fixes: #7416

• 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.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

Call out: I wasn't able to include a test because this requires an external SAT solver.

[codecov]

Codecov Report

Base: 78.46% // Head: 78.46% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (7b0b3e7) compared to base (6827ce5).
Patch coverage: 74.62% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7447      +/-   ##
===========================================
- Coverage    78.46%   78.46%   -0.01%     
===========================================
  Files         1662     1662              
  Lines       190904   190903       -1     
===========================================
- Hits        149794   149793       -1     
  Misses       41110    41110              

Impacted Files	Coverage Δ
...src/java_bytecode/java_bytecode_convert_method.cpp	97.67% <ø> (ø)
jbmc/src/java_bytecode/java_bytecode_language.cpp	92.60% <ø> (ø)
src/analyses/invariant_set.cpp	0.00% <0.00%> (ø)
src/ansi-c/ansi_c_declaration.h	95.00% <ø> (ø)
src/ansi-c/c_typecast.h	75.00% <ø> (ø)
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/cprover/simplify_state_expr.cpp	91.59% <ø> (ø)
src/goto-cc/as86_cmdline.cpp	0.00% <0.00%> (ø)
src/goto-cc/as_cmdline.cpp	0.00% <0.00%> (ø)
src/goto-cc/as_mode.cpp	0.00% <0.00%> (ø)
... and 199 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.