Various refactorings to clean up nll diagnostics

[mark-i-m]

• Create ErrorReportingCtx and ErrorConstraintInfo, vasting reducing the
  number of arguments passed around everywhere in the error reporting code
• Create RegionErrorNamingCtx, making a given lifetime have consistent
  numbering thoughout all error messages for that MIR def.
• Make the error reporting code return the DiagnosticBuilder rather than
  directly buffer the Diagnostic. This makes it easier to modify the
  diagnostic later, e.g. to add suggestions.

r? @estebank

Split out from #58281

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-09-12T22:05:30.4414447Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-12T22:05:30.4605984Z ##[command]git config gc.auto 0
2019-09-12T22:05:30.4685794Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-12T22:05:30.4762415Z ##[command]git config --get-all http.proxy
2019-09-12T22:05:30.4902419Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/64416/merge:refs/remotes/pull/64416/merge
---
2019-09-12T22:15:05.6118839Z     Checking rustc_mir v0.0.0 (/checkout/src/librustc_mir)
2019-09-12T22:15:06.4068533Z error[E0432]: unresolved import `crate::borrow_check::nll::region_infer::error_reporting::outlives_suggestion`
2019-09-12T22:15:06.4068969Z   --> src/librustc_mir/borrow_check/nll/region_infer/mod.rs:14:36
2019-09-12T22:15:06.4069259Z    |
2019-09-12T22:15:06.4069577Z 14 |     region_infer::error_reporting::outlives_suggestion::OutlivesSuggestionBuilder,
2019-09-12T22:15:06.4070298Z    |                                    ^^^^^^^^^^^^^^^^^^^ could not find `outlives_suggestion` in `error_reporting`
2019-09-12T22:15:06.7763970Z error[E0425]: cannot find value `errors_buffer` in this scope
2019-09-12T22:15:06.7764570Z     --> src/librustc_mir/borrow_check/nll/region_infer/mod.rs:1497:19
2019-09-12T22:15:06.7765450Z      |
2019-09-12T22:15:06.7765450Z      |
2019-09-12T22:15:06.7766179Z 1497 |         db.buffer(errors_buffer);
2019-09-12T22:15:06.7766701Z 
2019-09-12T22:15:09.4752006Z     Checking rustc_lint v0.0.0 (/checkout/src/librustc_lint)
2019-09-12T22:15:11.1921556Z     Checking rustc_passes v0.0.0 (/checkout/src/librustc_passes)
2019-09-12T22:15:11.9414770Z     Checking rustc_codegen_utils v0.0.0 (/checkout/src/librustc_codegen_utils)
---
2019-09-12T22:15:16.2336918Z == clock drift check ==
2019-09-12T22:15:16.2358406Z   local time: Thu Sep 12 22:15:16 UTC 2019
2019-09-12T22:15:16.5142957Z   network time: Thu, 12 Sep 2019 22:15:16 GMT
2019-09-12T22:15:16.5152159Z == end clock drift check ==
2019-09-12T22:15:17.6804657Z ##[error]Bash exited with code '1'.
2019-09-12T22:15:17.6838725Z ##[section]Starting: Checkout
2019-09-12T22:15:17.6840645Z ==============================================================================
2019-09-12T22:15:17.6840699Z Task         : Get sources
2019-09-12T22:15:17.6840748Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[mark-i-m]

Hmm... still running the tests, but it seems there might not be any changes because there are no outlives suggestions that might force a region number to be assigned earlier.

[estebank]

@bors r+

[bors]

📌 Commit 2a774b1 has been approved by estebank