SYNTHESIZER: Handle the case of violation in loop guards

[qinheping]

In current loop-invariant synthesizer, we synthesize two kinds of clauses separately.

1. in_clause that should hold when we enter the loop body---when the loop guard holds, that is loop_guard -> in_clause.
2. pos_clause that should hold when we leave the loop---when the loop guard doesn't hold, that is !loop_guard -> pos_clause.

We synthesize in_clause when the violations happen in the loop body. We synthesize pos_clause when the violation happen after the loop.

This PR handles the case that the violations happen in the loop guard (loop_contracts_synthesis_06/main.c as an example). In the case, the new clause we synthesize should hold for both of the cases that loop guard holds and loop guards doesn't hold.

We introduce a new variable violation_location to indicate whether the violation happen in the loop, after the loop, or in the loop guard, and add the new synthesized clause as in_clause, pos_clause, or both, respectively.

• 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.
• 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

Base: 78.48% // Head: 78.48% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (43f71f3) compared to base (531e768).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7457   +/-   ##
========================================
  Coverage    78.48%   78.48%           
========================================
  Files         1663     1663           
  Lines       191112   191136   +24     
========================================
+ Hits        149992   150017   +25     
+ Misses       41120    41119    -1     

Impacted Files	Coverage Δ
src/goto-synthesizer/cegis_verifier.h	100.00% <ø> (ø)
src/goto-synthesizer/cegis_verifier.cpp	87.97% <100.00%> (+0.79%)	⬆️
...hesizer/enumerative_loop_contracts_synthesizer.cpp	93.75% <100.00%> (+1.31%)	⬆️
src/goto-programs/goto_inline.cpp	95.83% <0.00%> (-0.84%)	⬇️
src/goto-programs/link_to_library.cpp	100.00% <0.00%> (ø)

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