When building with /Od - default cmake debug build for me, the __assume(false); trick does not work to get rid of the C4714 warnings

https://godbolt.org/z/a6xGnfP7d

D:\tmp\cppcheck\lib\keywords.cpp(205): warning C4715: 'Keywords::getOnly': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(226): warning C4715: 'Keywords::getOnly': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(168): warning C4715: 'Keywords::getAll': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(188): warning C4715: 'Keywords::getAll': not all control paths return a value

Proposed fix: also define NORETURN to [[noreturn]] when according to __has_cpp_attribute [[noreturn]] is supported https://en.cppreference.com/w/cpp/feature_test

(For previous discussion see also danmar#5497)

Moved __has_cpp_attribute block up to first choice. When we have a portable standard way of expressing it, that's the best choice.
(Addresses Review Feedback from Firewave)