Function string_cache_lookup_id 

pub fn string_cache_lookup_id(
    string_cache: &mut HashMap<String, u32, FxBuildHasher>,
    string_cache_inv: &mut Vec<Rc<String>>,
    s: &str,
) -> u32

Look up a string in the string interning cache, but skip the Rc::clone() overhead because we only need the ID.