syntax: improve message for misused inner attributes

for the code:
```
use std::io;

#![crate_type="rlib"] // ERROR: an inner attribute is not permitted in this context

fn say_hello() {
	println!("hello");
}
```

this PR provides another note to help programmer fixing this error more easily:
```
hello.rs:6:3: 6:4 error: an inner attribute is not permitted in this context
hello.rs:6 #![crate_type="rlib"]
             ^
hello.rs:6:3: 6:4 note: put inner attribute in top of file or block
hello.rs:6 #![crate_type="rlib"]
             ^
```
I think this should also be a `span_help` instead of a `span_note`.