split infer/error_reporting to separate modules

[cengiz-io]

Hello!

I tried to make librustc/infer/error_reporting,rs more readable by separating it into modules based on traits that they're focused on.

It might not be the optimal separation criteria for those functions, but I've yet to determine a general pattern between them, So, if you have any suggestions, please send it right away! 🚀

Thanks goes to @nikomatsakis for supporting.

Have a nice one!

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

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[bors]

☔ The latest upstream changes (presumably #40091) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

r? @nikomatsakis