"Duplicate method" error should point to previous definition

[jonas-schievink]

(same goes for "duplicate associated function" and similar errors)

I was playing around with macro-generated methods, and stumbled upon a bad diagnostic:

struct S;

macro_rules! methods {
    ( $($name:ident)* ) => {
        $( fn $name(&self) {} )*
    }
}

impl S {
    fn a(&self) {}

    methods!(a b c);
}

fn main() {}

gives:

<anon>:5:12: 5:30 error: duplicate method [E0201]
<anon>:5         $( fn $name(&self) {} )*
                    ^~~~~~~~~~~~~~~~~~
<anon>:12:5: 12:21 note: in this expansion of methods! (defined in <anon>)

Rustc should print a note: previous definition here and include the name of the method in the diagnostic (since both could be defined by macros, in which case the name will not be obvious).