C++ front-end: fix type checking of anonymous bit fields

[tautschnig]

For unsigned : 2; we tried to merge a necessarily empty (not nil!) irept via merge_types. MacOS has such anonymous bit fields in some system headers.

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• n/a 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]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (fa1d719) 78.74% compared to head (be2a41c) 78.74%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7848      +/-   ##
===========================================
- Coverage    78.74%   78.74%   -0.01%     
===========================================
  Files         1699     1699              
  Lines       195105   195105              
===========================================
- Hits        153634   153627       -7     
- Misses       41471    41478       +7     

Files Changed	Coverage Δ
src/cpp/parse.cpp	69.39% <100.00%> (+0.30%)	⬆️

... and 13 files with indirect coverage changes

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