CONTRACTS: use vars auxiliary for DFCC

[remi-delmas-3000]

The is_auxiliary flag is now set for instrumentation variables created for DFCC. This will reduce noise in counter examples.

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[codecov]

Codecov Report

Base: 78.28% // Head: 78.28% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ac3e40c) compared to base (a295651).
Patch coverage: 81.81% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7326   +/-   ##
========================================
  Coverage    78.28%   78.28%           
========================================
  Files         1642     1642           
  Lines       190004   190013    +9     
========================================
+ Hits        148744   148758   +14     
+ Misses       41260    41255    -5     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-programs/graphml_witness.cpp	52.94% <0.00%> (-0.15%)	⬇️
src/goto-programs/interpreter.cpp	52.38% <0.00%> (+0.17%)	⬆️
src/util/expr_initializer.cpp	85.10% <80.00%> (+1.77%)	⬆️
jbmc/src/java_bytecode/expr2java.cpp	86.25% <93.75%> (ø)
src/ansi-c/c_typecheck_initializer.cpp	76.57% <100.00%> (+0.04%)	⬆️
src/ansi-c/expr2c.cpp	67.33% <100.00%> (-0.02%)	⬇️
src/goto-instrument/contracts/contracts.cpp	95.09% <100.00%> (+0.06%)	⬆️
...instrument/contracts/dynamic-frames/dfcc_utils.cpp	95.76% <100.00%> (+0.03%)	⬆️
src/goto-instrument/dump_c.cpp	79.97% <100.00%> (-0.03%)	⬇️
... and 4 more

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