C++ front-end: support attributes with tag-less structs

[tautschnig]

Seen in GCC's STL.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.64%. Comparing base (0889cfa) to head (fc31dfc).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8289      +/-   ##
===========================================
+ Coverage    77.55%   77.64%   +0.09%     
===========================================
  Files         1721     1721              
  Lines       190128   189932     -196     
  Branches     18438    18436       -2     
===========================================
+ Hits        147448   147471      +23     
+ Misses       42680    42461     -219     

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