argc/argv modelling: argument string must remain in scope

[tautschnig]

We previously produced an argument string that was marked DEAD before entering main, making accesses to argv[i] ones to a dead object.

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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).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5d85d38) 79.69% compared to head (ab93890) 79.69%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8174   +/-   ##
========================================
  Coverage    79.69%   79.69%           
========================================
  Files         1680     1680           
  Lines       195242   195245    +3     
========================================
+ Hits        155604   155607    +3     
  Misses       39638    39638           

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