Merge #5541

5541: add completion for rustc lints r=matklad a=Emilgardis

This is a very naive approach to provide completions for lints.

Preferably, this would pull from current `rustup which rustc` via `rustc -W help <crate>`, but currently `rustc` only provides human output for lints.

![image](https://user-images.githubusercontent.com/1502855/88492913-90dec880-cfae-11ea-89d8-2b494951b20c.png)

also, clippy would be really nice but I feel like for that ra should really pull from clippy


Co-authored-by: Emil Gardström <[email protected]>

Author: bors[bot]