Consolidate region error reporting in `rustc_infer`
More work on https://github.com/rust-lang/rust/issues/127492. Separate but important step, since I'm gonna likely pull everything else here into another module.
I don't think I'm confident whether `nice_region_error` should be a submodule of the new `rustc_infer::infer::error_reporting::region` module, so I left it alone for now.
r? lcnr
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