Rollup merge of #40128 - cengizIO:master, r=nikomatsakis

Move two large error_reporting fn's to a separate file

Hello!

I tried to make `librustc/infer/error_reporting,rs` more readable by modularizing it and moving its two largest functions to a separate file.

If you have any suggestions, please send it right away! 🚀

Thanks goes to @nikomatsakis for supporting.

Author: frewsxcv