Add an API to check if a function is intrinsic

[momvart]

Can we have the following API in FnDef?

impl FnDef {
    pub fn is_intrinsic(&self) -> bool {
        with(|cx| cx.is_intrinsic(*self))
    }
}

and

pub trait Context {
    fn is_intrinsic(&self, def: FnDef) -> bool;
}

[momvart]

@rustbot claim