Warn when control flow reaches end of a `_Noreturn` function

[michael-schwarz]

Since https://github.com/goblint/cil/pull/58, CIL supports the _Noreturn attribute.

As remarked by @sim642 on Slack, we should also do something with _Noreturn in Goblint and emit a warning when control flow does indeed reach the end of such a function (i.e. return node is not dead).

This probably also requires investigating which standard library functions terminate execution (e.g. exit(...) and friends).

[sim642]

I think we already handle the usual suspects as non-returning: https://github.com/goblint/analyzer/blob/e81b1a147f420983583e680a57d8a6d8b071ca68/src/analyses/base.ml#L2188-L2205

Of course I guess a modern standard library header would also have them with _Noreturn (or equivalent). Actually it might be even worth having a sem option, whether to trust _Noreturn attributes (and make them all raise Deadcode just based on that). While doing that we could still warn about control flow reaching the end of the function within its implementation. This might avoid a cascade of _Noreturn warnings if we don't handle the innermost root cause.