[significant_drop_tightening] Functions that return literals should not trigger the lint

[c410-f3r]

Literals are cheap expressions that won't interfere with the release of a lock.

pub fn foo() -> bool {
    let mutex = Mutex::new(1i32);
    let lock = mutex.lock().unwrap();
    let _ = lock.abs();
    true
}

cc #11125 (comment)