External SAT back-end: clear clauses after writing DIMACS

[tautschnig]

There is no need to hold on to clauses. Freeing up memory makes it available to the external SAT process instead.

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[codecov]

Codecov Report

Base: 78.28% // Head: 78.29% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (fb99063) compared to base (d7dd40a).
Patch coverage: 92.62% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7346   +/-   ##
========================================
  Coverage    78.28%   78.29%           
========================================
  Files         1642     1642           
  Lines       189995   190023   +28     
========================================
+ Hits        148737   148769   +32     
+ Misses       41258    41254    -4     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_byte_extract.cpp	69.73% <ø> (ø)
src/solvers/flattening/boolbv_byte_update.cpp	100.00% <ø> (ø)
src/solvers/flattening/boolbv_equality.cpp	63.63% <ø> (ø)
src/solvers/flattening/boolbv_index.cpp	72.91% <ø> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	96.81% <ø> (ø)
src/util/byte_operators.h	92.72% <ø> (ø)
src/cprover/propagate.cpp	65.85% <16.66%> (-8.44%)	⬇️
src/util/lower_byte_operators.cpp	92.70% <95.50%> (ø)
src/solvers/sat/external_sat.cpp	88.23% <100.00%> (+0.14%)	⬆️
src/solvers/smt2/smt2_conv.cpp	66.79% <100.00%> (+0.40%)	⬆️
... and 3 more

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