librustc_errors: Extract sugg/subst handling into method

[phansch]

An initial refactoring before working on #61809.

This moves the whole block into a method so that it can be reused in the
annotate-snippet emitter. The method is already used in the new emitter, but
there's no UI tests with suggestions included in this PR.

A first look at some UI tests with suggestions showed that there's some
more work to do in annotate-snippet-rs before the new output is closer to the
current one, so I opted to do that in a second step.

r? @estebank

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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2019-09-01T10:38:58.4577115Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-01T10:38:58.4777073Z ##[command]git config gc.auto 0
2019-09-01T10:38:58.4863700Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-01T10:38:58.4940699Z ##[command]git config --get-all http.proxy
2019-09-01T10:38:58.5070235Z ##[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/64058/merge:refs/remotes/pull/64058/merge
---
2019-09-01T10:45:47.3218053Z    Compiling serde_json v1.0.40
2019-09-01T10:45:49.1574047Z    Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
2019-09-01T10:46:00.1333529Z     Finished release [optimized] target(s) in 1m 29s
2019-09-01T10:46:00.1409836Z tidy check
2019-09-01T10:46:00.4952235Z tidy error: /checkout/src/librustc_errors/emitter.rs:205: line longer than 100 chars
2019-09-01T10:46:02.0958885Z some tidy checks failed
2019-09-01T10:46:02.8902564Z 
2019-09-01T10:46:02.8902564Z 
2019-09-01T10:46:02.8903667Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor"
2019-09-01T10:46:02.8905034Z 
2019-09-01T10:46:02.8905139Z 
2019-09-01T10:46:02.8905336Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-09-01T10:46:02.8905696Z Build completed unsuccessfully in 0:01:33
2019-09-01T10:46:02.8905696Z Build completed unsuccessfully in 0:01:33
2019-09-01T10:46:02.8905889Z == clock drift check ==
2019-09-01T10:46:02.8905966Z   local time: Sun Sep  1 10:46:02 UTC 2019
2019-09-01T10:46:02.8906042Z   network time: Sun, 01 Sep 2019 10:46:02 GMT
2019-09-01T10:46:02.8906089Z == end clock drift check ==
2019-09-01T10:46:03.6935872Z ##[error]Bash exited with code '1'.
2019-09-01T10:46:03.6967378Z ##[section]Starting: Checkout
2019-09-01T10:46:03.6968914Z ==============================================================================
2019-09-01T10:46:03.6968984Z Task         : Get sources
2019-09-01T10:46:03.6969031Z 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)

[estebank]

@bors r+

[bors]

📌 Commit 84ccbe2 has been approved by estebank