Dead code should consider lang items live

[huonw]

They don't get used in the AST directly, so the dead code lint can't detect them.

#[no_std];
#[no_main];

#[no_mangle]
pub fn main(_: i32, _: **u8) -> i32 {
    1 / 1
}

#[lang="fail_"]
fn fail(_: *u8, _: *u8, _: uint) -> ! { loop {} }

$ rustc lang-dead.rs
lang-dead.rs:10:1: 10:50 warning: code is never used: `fail`, #[warn(dead_code)] on by default
lang-dead.rs:10 fn fail(_: *u8, _: *u8, _: uint) -> ! { loop {} }
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[... linking errors ...]