Rollup merge of rust-lang#39977 - frewsxcv:error-reporting-cleanup, r=eddyb

librustc error_reporting.rs cleanup.

Read some code in librustc, mainly in error_reporting.rs, and cleaned up some things along the way. I recommend looking at each commit individually or looking at the [whitespace insensitive diff](https://github.com/rust-lang/rust/pull/39977/files?w=1).

Author: eddyb