Default initialize class_hierarchy

[romainbrenguier]

This could prevent compilation in some compiler configurations.

• 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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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 2568e40).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/118088141

[codecov-io]

Codecov Report

Merging #4876 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4876   +/-   ##
========================================
  Coverage    69.09%   69.09%           
========================================
  Files         1298     1298           
  Lines       106947   106947           
========================================
  Hits         73896    73896           
  Misses       33051    33051

Impacted Files	Coverage Δ
...de/java_bytecode_convert_method/convert_method.cpp	100% <ø> (ø)	⬆️

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0cc6f93).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/118239674