CHC encoder: universally quantify nondeterministic choices

[kroening]

Nondeterministic choices must be universally quantified, as opposed to
existentially, to yield the desired adversarial nondeterminism.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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

Merging #6958 (8ce6943) into develop (a1ecd93) will increase coverage by 0.00%.
The diff coverage is 78.07%.

@@           Coverage Diff            @@
##           develop    #6958   +/-   ##
========================================
  Coverage    77.88%   77.88%           
========================================
  Files         1569     1569           
  Lines       180894   180989   +95     
========================================
+ Hits        140882   140962   +80     
- Misses       40012    40027   +15     

Impacted Files	Coverage Δ
src/solvers/smt2_incremental/object_tracking.h	100.00% <ø> (ø)
src/util/dstring.h	100.00% <ø> (ø)
src/util/std_expr.h	92.09% <0.00%> (-0.91%)	⬇️
src/solvers/prop/prop_conv_solver.cpp	87.50% <16.66%> (-1.65%)	⬇️
src/solvers/smt2/smt2_conv.cpp	68.76% <16.66%> (-0.11%)	⬇️
src/solvers/flattening/boolbv.cpp	87.08% <25.00%> (-0.76%)	⬇️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	88.56% <92.85%> (+0.26%)	⬆️
src/goto-instrument/horn_encoding.cpp	73.24% <100.00%> (+0.53%)	⬆️
src/solvers/smt2_incremental/object_tracking.cpp	98.41% <100.00%> (+0.41%)	⬆️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.68% <100.00%> (+<0.01%)	⬆️
... and 9 more

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