Auto merge of #45752 - estebank:highlight-primary, r=nikomatsakis

Highlight code on diagnostics when underlined

Highlight the label's span with the respective color:

<img width="692" alt="" src="https://user-images.githubusercontent.com/1606434/32411026-a1842482-c18d-11e7-9933-6510eefbad19.png">

Fix #42112.

diff --cc src/librustc_driver/lib.rs
Simple merge

diff --cc src/librustc_errors/emitter.rs
Simple merge

diff --cc src/librustdoc/test.rs
Simple merge