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.

src/test/compile-fail/lint-uppercase-variables.rs

@@ -33,7 +33,8 @@ fn main() {
     match f.read(&mut buff) {
         Ok(cnt) => println!("read this many bytes: {}", cnt),
         Err(IoError{ kind: EndOfFile, .. }) => println!("Got end of file: {}", EndOfFile.to_string()),
-        //~^ ERROR variable `EndOfFile` should have a snake case name such as `end_of_file`
+//~^ ERROR variable `EndOfFile` should have a snake case name such as `end_of_file`
+//~^^ WARN `EndOfFile` is named the same as one of the variants of the type `std::io::IoErrorKind`
     }
 
     test(1);