This is to indicate when a code path is absolutely, for certain, 100%
unreachable. Useful as the default case inside a case-switch that is for
sure 100% exhaustive because it's inside the case of another case-switch
(and the default case is there to suppress compiler warnings about the
case-switch not being exhaustive), which is a situation coming up in my
scriptclass::startgamemode refactor.
It does this by deliberately invoking undefined behavior, either using a
compiler builtin that does the same thing or being a noreturn function
that returns. (And undefined behavior is not undefined behavior if it is
not executed in a code path, otherwise all NULL checks would be useless
because it'd dereference something that could be NULL in another code
path.)
