C front-end: recognize __builtin_inff() as constant expression

[tautschnig]

This is accepted as a constant expression by GCC, so we need to mimic this to successfully type check code that relies on this.

Fixes: #8710

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.94%. Comparing base (8640fc9) to head (2cef358).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8775   +/-   ##
========================================
  Coverage    79.93%   79.94%           
========================================
  Files         1698     1698           
  Lines       187698   187703    +5     
  Branches        73       73           
========================================
+ Hits        150034   150050   +16     
+ Misses       37664    37653   -11     

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[kroening]

May I suggest naming the test regression/cbmc/builtin_inf/inff_static_init.c

[tautschnig]

May I suggest naming the test regression/cbmc/builtin_inf/inff_static_init.c

Done!