Merge #6734

Merge #6734

6734: Emit additional diagnostics for hints/help/etc r=lnicola a=jonas-schievink

This makes rust-analyzer diagnostics match native rustc diagnostics in the terminal more closely. Unfortunately all of this is a bodge, since we already provide this information to the client in the form of `DiagnosticRelatedInformation`, but at least VS Code chooses such a poor UI for these that they don't help much, as evidenced [here](https://twitter.com/yaahc_/status/1335297260444250112) and in https://github.com/rust-lang/rust/issues/79741.

This PR papers over these client UI problems by taking the `DiagnosticRelatedInformation` and turning each one into its own hint-level diagnostic, which makes it show up in the source code. Quick fixes are attached to all resulting diagnostics, which makes them more discoverable.

### Example: "Consider removing this semicolon"

![screenshot-2020-12-06-01:27:29](https://user-images.githubusercontent.com/1786438/101268366-46423980-3762-11eb-9a69-1ff0b1806c2f.png)
![screenshot-2020-12-06-01:27:39](https://user-images.githubusercontent.com/1786438/101268367-46dad000-3762-11eb-81fa-afd234d44f17.png)
![screenshot-2020-12-06-01:27:46](https://user-images.githubusercontent.com/1786438/101268368-46dad000-3762-11eb-9205-4b9bd9f4406d.png)

### Example: "Value used after move"

![screenshot-2020-12-06-01:33:00](https://user-images.githubusercontent.com/1786438/101268447-22332800-3763-11eb-85ce-8c742927a2c8.png)
![screenshot-2020-12-06-01:33:07](https://user-images.githubusercontent.com/1786438/101268448-22cbbe80-3763-11eb-8f16-0590895d8bc6.png)

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>