"discriminant value already exists" message for overlapping C-like enum variants could be tweaked to be clearer

[huonw]

The error message could:

• mention the value in question (i.e. 1 in the examples below), since the conflicting variant doesn't necessarily have an literal discriminant (D and E below)
• point to the other overlapping variant(s)

static N: int = 1;

enum Foo {
    A = 1,
    B = 1,
    C = 0,
    D,
    E = N,
}

fn main() {}

<anon>:5:5: 5:10 error: discriminant value already exists
<anon>:5     B = 1,
             ^~~~~
<anon>:7:5: 7:6 error: discriminant value already exists
<anon>:7     D,
             ^
<anon>:8:5: 8:10 error: discriminant value already exists
<anon>:8     E = N,
             ^~~~~