Support anonymous struct embedding via -fms-extensions

[chaosape]

When -fms-extensions is passed, tagged struct/union members without a declarator name are treated as anonymous embedding (Plan 9 C extension). This allows code using the GCC extension where 'struct tag;' inside another struct inlines the tagged struct's fields.

The implementation lets the tagged anonymous member fall through to the existing anonymous member handling (the $anon naming + set_anonymous pass), rather than skipping it with 'continue'.

Only this one behavior is implemented — no other MSVC or Plan 9 extensions. The internal config field is named
allow_anonymous_struct_embedding to be explicit about scope.

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• 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 is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.59%. Comparing base (808dae2) to head (c0a1b97).
⚠️ Report is 25 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/goto-cc/gcc_mode.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9022      +/-   ##
===========================================
+ Coverage    80.55%   80.59%   +0.04%     
===========================================
  Files         1707     1711       +4     
  Lines       189051   189458     +407     
  Branches        73       73              
===========================================
+ Hits        152299   152703     +404     
- Misses       36752    36755       +3     

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