C/C++ front-ends: report a meaningful error for invalid function definition

[tautschnig]

The parser happily accepts the syntax shown in the regression test
included, but type checking strictly expects code instead of an
initializer list. Report a proper error message, like GCC does, instead
of failing an invariant.

Test generated using C-Reduce starting from an SV-COMP task.

• 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 #5674 (5233796) into develop (e23bf5c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5674   +/-   ##
========================================
  Coverage    69.45%   69.46%           
========================================
  Files         1243     1243           
  Lines       100614   100621    +7     
========================================
+ Hits         69886    69893    +7     
  Misses       30728    30728           

Flag	Coverage Δ
cproversmt2	43.14% <22.22%> (-0.01%)	⬇️
regression	66.35% <100.00%> (+<0.01%)	⬆️
unit	32.28% <22.22%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	81.52% <100.00%> (+0.20%)	⬆️
src/cpp/cpp_typecheck_function.cpp	94.82% <100.00%> (+0.38%)	⬆️

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[martin-cs]

"Giving a helpful error message rather than an assertion failure on user input" is a great look!