add `MISSING_HEADER` hook

[firewave]

Adding a hook which provides data when MISSING_HEADER is encountered would allow Cppcheck a way to inject the macros the header defines without providing the whole code - see https://trac.cppcheck.net/ticket/13341. It would also allow for on-demand automatic library loading.

To work correctly this would also need to provide some context since the header evaluation is based on previously defines macros.

[firewave]

This could probably be implemented via the FileDataCache by providing a function an virtual function.