C front-end: ignore or reject excess union initializers

[tautschnig]

Visual Studio treats excess union initializers as an error, for GCC and
Clang it's a warning. When just a warning, they are ignored. Make sure
we implement the same behaviour and do not wrongly overwrite the value.

• 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 #5702 (ae6107b) into develop (8ea67e7) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           develop    #5702      +/-   ##
===========================================
- Coverage    69.50%   69.50%   -0.01%     
===========================================
  Files         1243     1243              
  Lines       100674   100680       +6     
===========================================
+ Hits         69977    69981       +4     
- Misses       30697    30699       +2     

Flag	Coverage Δ
cproversmt2	43.15% <75.00%> (+<0.01%)	⬆️
regression	66.40% <75.00%> (+<0.01%)	⬆️
unit	32.25% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_initializer.cpp	75.88% <75.00%> (-0.16%)	⬇️

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