Warn on pattern bindings that have the same name as a variant

...of the type being matched.

This change will result in a better diagnostic for code like the following:

```rust
enum Enum {
    Foo,
    Bar
}

fn f(x: Enum) {
    match x {
        Foo => (),
        Bar => ()
    }
}
```

which would currently simply fail with an unreachable pattern error
on the 2nd arm.

The user is advised to either use a qualified path in the patterns
or import the variants explicitly into the scope.