disallow flexible array members in unions

[kroening]

This change yields an error if a flexible array member is used in a union. C11 allows them in structs, but not unions.

• 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.
• 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.

[tautschnig]

This is failing the windows_h_VS_2012 and windows_h_VS_2013 regression tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (66f8c1b) to head (eccad80).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7296      +/-   ##
===========================================
- Coverage    80.55%   80.55%   -0.01%     
===========================================
  Files         1707     1707              
  Lines       189012   189016       +4     
  Branches        73       73              
===========================================
+ Hits        152257   152258       +1     
- Misses       36755    36758       +3     

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