Rename invalid false cfg to valid _false cfg

While `false` is accepted by `--cfg` it isn't by `#[cfg(false)]`
since in that context `false` is the boolean not a ident.

tests/ui/instrument-coverage/off-values.rs

@@ -1,9 +1,9 @@
 //@ check-pass
-//@ revisions: n no off false zero
+//@ revisions: n no off _false zero
 //@ [n] compile-flags: -Cinstrument-coverage=n
 //@ [no] compile-flags: -Cinstrument-coverage=no
 //@ [off] compile-flags: -Cinstrument-coverage=off
-//@ [false] compile-flags: -Cinstrument-coverage=false
+//@ [_false] compile-flags: -Cinstrument-coverage=false
 //@ [zero] compile-flags: -Cinstrument-coverage=0
 
 fn main() {}