C front-end: typecheck conditional operator over string literal and 0

[tautschnig]

0 is a valid null-pointer constant, even in the absence of an explicit type cast.

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[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.31%. Comparing base (8646bfa) to head (0f14879).
Report is 4 commits behind head on develop.

Files	Patch %	Lines
src/ansi-c/c_typecheck_expr.cpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7946      +/-   ##
===========================================
- Coverage    78.37%   78.31%   -0.07%     
===========================================
  Files         1726     1726              
  Lines       188322   188477     +155     
  Branches     18460    18441      -19     
===========================================
+ Hits        147606   147611       +5     
- Misses       40716    40866     +150     

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[kroening]

We may wish to have a function or method for the decay of the string literal type.