[CONTRACTS] Use fresh converter in loop contracts instrumentation

[qinheping]

Use fresh converter during loop contracts instrumentation to make sure the jumps in the loop invariants will be correctly directed.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.21%. Comparing base (f18b509) to head (7273894).
Report is 28 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8282      +/-   ##
===========================================
+ Coverage    77.55%   78.21%   +0.66%     
===========================================
  Files         1721     1722       +1     
  Lines       190128   188648    -1480     
  Branches     18438    18452      +14     
===========================================
+ Hits        147448   147547      +99     
+ Misses       42680    41101    -1579     

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[tautschnig]

Could you please add a test to demonstrate what difference this makes?

[tautschnig]

Please remove #include <ansi-c/goto-conversion/goto_convert_class.h> from this file and move it to the .cpp file.

[qinheping]

Could you please add a test to demonstrate what difference this makes?

The test will be loop invariants as statement expressions containing GOTO statements, such as

int main()
{
  unsigned char *i = malloc(5);

  while(i != i + 5)
    __CPROVER_loop_invariant(({
      goto label;
      unsigned b = 1 == 1;
    label:
      b;
    }))
    {
      const char lower = *i++;
    }
}

However, such loop invariant doesn't typecheck now as statement expression are side-effect expression, even thought this loop invariant is side-effect free. I will refactor #8281 soon to allow such side-effect free statement expression, and add the test in #8281.