The fail macro defines some function/static items internally, which got
a dead_code warning when `fail!()` is used inside a dead function. This
is ugly and unnecessarily reveals implementation details, so the
warnings can be squashed.
Fixes#16192.
This generalises the behaviour with struct fields (which recieve no
dead_code warning if they have a leading _), and other similar lints, to
all items, e.g. `fn _foo() {} fn main() {}` has no warnings.
