Auto merge of rust-lang#17253 - ThouCheese:master, r=Veykril

Implement assist to switch between doc and normal comments

Hey first PR to rust-analyzer to get my feet wet with the code base. It's an assist to switch a normal comment to a doc comment and back, something I've found myself doing by hand a couple of times.

I shamelessly stole `relevant_line_comments` from `convert_comment_block`, because I didn't see any inter-assist imports happening in the files I peeked at so I thought this would be preferable.

Author: bors