Make prefix filter operator const [TG-8603]

[peterschrammel]

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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-io]

Codecov Report

Merging #4971 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4971      +/-   ##
===========================================
+ Coverage    69.24%   69.27%   +0.03%     
===========================================
  Files         1309     1307       -2     
  Lines       108453   108143     -310     
===========================================
- Hits         75096    74916     -180     
+ Misses       33357    33227     -130

Impacted Files	Coverage Δ
src/util/prefix_filter.h	100% <ø> (ø)	⬆️
src/util/prefix_filter.cpp	100% <100%> (ø)	⬆️
src/goto-symex/renaming_level.cpp	92.53% <0%> (-2.52%)	⬇️
src/statement-list/statement_list_entry_point.cpp	86.36% <0%> (-1.52%)	⬇️
src/util/expr_cast.h	89.47% <0%> (-1.44%)	⬇️
src/statement-list/scanner.l	76.44% <0%> (-1.39%)	⬇️
src/cpp/cpp_declarator_converter.cpp	76.06% <0%> (-0.99%)	⬇️
jbmc/src/java_bytecode/java_bytecode_parser.cpp	89.59% <0%> (-0.98%)	⬇️
src/goto-symex/goto_symex.cpp	96.42% <0%> (-0.35%)	⬇️
jbmc/unit/java-testing-utils/require_type.cpp	89.65% <0%> (-0.28%)	⬇️
... and 56 more

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 64c16fa).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/121490558
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.