Auto merge of #55432 - zackmdavis:single_life, r=nikomatsakis

single life

 * structured ~~autofixable~~ (well, pending #53934 and rust-lang-nursery/rustfix#141) suggestions for the single-use-lifetimes lint in the case of function and method reference args
 * don't consider the anonymous lifetime `'_` as "single-use" (it's intended for exactly this sort of thing)

![single_life](https://user-images.githubusercontent.com/1076988/47613227-3b2b6400-da48-11e8-8efd-cb975ddf537d.png)

r? @nikomatsakis

diff --cc src/librustc/middle/resolve_lifetime.rs
Simple merge