Rollup merge of rust-lang#23782 - mvdnes:obsolete_note, r=alexcrichton

 When emmitting a note, previously it was not known if the note was for an error or a
warning. If it was for a warning, then with `-Awarnings` it should not have been print.
The `emit_for` function allows someone to specify which level should determine its visibility.

An example:
```rust
extern crate \"std\" as std2;
fn main() {}
```

When compiling with `-Awarnings`, this would previously emit `note: use an identifier not in quotes instead` (and nothing else).
With this patch, it will be completely silent as expected.