Fix statement-expression expansion for Kani-provided quantifiers

[tautschnig]

CBMC side of model-checking/kani#4020: re-using the same converter instance would confuse finish-gotos (when really we don't want gotos inside the statement expression to be considered at all by the main goto-converter instances).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (34dc8dc) to head (f77349f).
Report is 37 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8649   +/-   ##
========================================
  Coverage    80.36%   80.36%           
========================================
  Files         1686     1686           
  Lines       206981   206983    +2     
  Branches        73       73           
========================================
+ Hits        166337   166342    +5     
+ Misses       40644    40641    -3     

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[kroening]

CBMC side of model-checking/kani#4020: re-using the same converter instance would confuse finish-gotos (when really we don't want gotos inside the statement expression to be considered at all by the main goto-converter instances).

Did you try that with gcc/clang?

[kroening]

E.g., clang will accept

#include <stdio.h>

int main()
{
  int x = 1;
  x = ({ goto foo; 2; });
  foo:
  printf("x: %d\n", 1);
}

[tautschnig]

E.g., clang will accept

#include <stdio.h>

int main()
{
  int x = 1;
  x = ({ goto foo; 2; });
  foo:
  printf("x: %d\n", 1);
}

While that is true for statement expressions in general we must not accept this for the use in quantifiers. As discussed offline a wider redesign is required in the medium term so as not to re-purpose statement expressions, but the PR at hand addresses the immediate bug.