Fix forward declarations handling

[bkryza]

Fixes #388

[codecov]

Codecov Report

Attention: Patch coverage is 96.22642% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.26%. Comparing base (ac667d5) to head (505e440).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
.../class_diagram/visitor/translation_unit_visitor.cc	96.22%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
+ Coverage   90.25%   90.26%   +0.01%     
==========================================
  Files         152      152              
  Lines       19031    19074      +43     
==========================================
+ Hits        17176    17217      +41     
- Misses       1855     1857       +2     

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