added constness_ptr as pointer wrapper to ensure actual method constness

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Taken from danmar/cppcheck#4785.

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I might spin out the class into a separate repo (as public domain) as this seems like something useful in general.

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This fails to compile with GCC before 15:

simplecpp.cpp:1742:37: error: operands to ‘?:’ have different types ‘const simplecpp::constness_ptr<simplecpp::Token>’ and ‘std::nullptr_t’
 1742 |                 valueToken = argtok ? argtok->next : nullptr;
      |                              ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

And with Visual Studio:

     6>C:\a\simplecpp\simplecpp\simplecpp.cpp(1742,54): error C2446: ':': no conversion from 'nullptr' to 'const simplecpp::constness_ptr<simplecpp::Token>' [C:\a\simplecpp\simplecpp\simplecpp_obj.vcxproj]
             C:\a\simplecpp\simplecpp\simplecpp.cpp(1742,54):
             Constructor for class 'simplecpp::constness_ptr<simplecpp::Token>' is declared 'explicit'