C library: add __to{lower,upper} as alternative names for {tolower,toupper}

[tautschnig]

macOS eventually calls a function called __tolower (__toupper). Rename our previous implementation to __CPROVER_tolower (__CPROVER_toupper) and make both tolower (toupper) and __tolower (__toupper) use this. Also enable their regression tests.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.55%. Comparing base (f18b509) to head (98d09fb).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8283   +/-   ##
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  Coverage    77.55%   77.55%           
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  Files         1721     1721           
  Lines       190128   190128           
  Branches     18438    18438           
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  Hits        147448   147448           
  Misses       42680    42680           

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