Clone base decl when having an anonymous struct or union

[kchristin22]

Fixes #1151

When having an anonymous member expression, the declaration of its base is cloned instead of this empty decl.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.56%. Comparing base (b5e0d12) to head (d1f9559).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1152   +/-   ##
=======================================
  Coverage   94.56%   94.56%           
=======================================
  Files          51       51           
  Lines        8941     8944    +3     
=======================================
+ Hits         8455     8458    +3     
  Misses        486      486           

Files with missing lines	Coverage Δ
lib/Differentiator/ReverseModeVisitor.cpp	95.60% <100.00%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
lib/Differentiator/ReverseModeVisitor.cpp	95.60% <100.00%> (+<0.01%)	⬆️

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"