C/C++ front-end: do not accept void-typed objects or members

[tautschnig]

We silently accepted these, as many cases would work fine in goto
programs. The included regression test, however, demonstrates where this
would fail an invariant during Boolean flattening (if run with
--pointer-check). GCC refuses void typed variables, and so should we.

Test originates from an SV-COMP task, reduced to this minimal example
using C-Reduce.

• 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

Merging #5676 (9018306) into develop (4c2b413) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5676   +/-   ##
========================================
  Coverage    69.45%   69.46%           
========================================
  Files         1243     1243           
  Lines       100620   100635   +15     
========================================
+ Hits         69887    69902   +15     
  Misses       30733    30733           

Flag	Coverage Δ
cproversmt2	43.13% <13.33%> (-0.01%)	⬇️
regression	66.35% <100.00%> (+<0.01%)	⬆️
unit	32.27% <13.33%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	81.52% <100.00%> (+0.20%)	⬆️
src/ansi-c/c_typecheck_type.cpp	77.19% <100.00%> (+0.10%)	⬆️
src/cpp/cpp_typecheck_code.cpp	72.47% <100.00%> (+0.47%)	⬆️
src/cpp/cpp_typecheck_compound_type.cpp	85.57% <100.00%> (+0.07%)	⬆️
src/cpp/cpp_typecheck_declaration.cpp	73.84% <100.00%> (+1.26%)	⬆️

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[martin-cs]

If you added copies of the test to regression/cpp/ then you could get 100% diff coverage.

[tautschnig]

If you added copies of the test to regression/cpp/ then you could get 100% diff coverage.

Indeed, done! (Which of course required me expanding my changes in the C++ front-end - testing does help, after all ;-) )

[tautschnig]

#5679 needs to be merged first to make CI happy again.