Add E0734 and its long explanation

[GuillaumeGomez]

Part of #61137

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rust-highfive]

r? @estebank

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[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

2019-09-25T12:00:06.2748624Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-25T12:00:06.2930042Z ##[command]git config gc.auto 0
2019-09-25T12:00:06.3016422Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-25T12:00:06.3079358Z ##[command]git config --get-all http.proxy
2019-09-25T12:00:06.3224855Z ##[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/64763/merge:refs/remotes/pull/64763/merge
---
2019-09-25T12:07:32.8981132Z     Finished release [optimized] target(s) in 1m 32s
2019-09-25T12:07:32.9067197Z tidy check
2019-09-25T12:07:33.9818003Z * 579 error codes
2019-09-25T12:07:33.9818743Z * highest error code: E0734
2019-09-25T12:07:34.0806413Z tidy error: /checkout/src/librustc/error_codes.rs:2228: malformed stability attribute: can't parse `since` key
2019-09-25T12:07:35.0307682Z some tidy checks failed
2019-09-25T12:07:35.0313434Z 
2019-09-25T12:07:35.0313434Z 
2019-09-25T12:07:35.0315782Z 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"
2019-09-25T12:07:35.0318958Z 
2019-09-25T12:07:35.0319307Z 
2019-09-25T12:07:35.0333972Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-09-25T12:07:35.0334095Z Build completed unsuccessfully in 0:01:35
2019-09-25T12:07:35.0334095Z Build completed unsuccessfully in 0:01:35
2019-09-25T12:07:35.0385728Z == clock drift check ==
2019-09-25T12:07:35.0407283Z   local time: Wed Sep 25 12:07:35 UTC 2019
2019-09-25T12:07:35.1902227Z   network time: Wed, 25 Sep 2019 12:07:35 GMT
2019-09-25T12:07:35.1906695Z == end clock drift check ==
2019-09-25T12:07:36.5107261Z ##[error]Bash exited with code '1'.
2019-09-25T12:07:36.5138556Z ##[section]Starting: Checkout
2019-09-25T12:07:36.5140368Z ==============================================================================
2019-09-25T12:07:36.5140455Z Task         : Get sources
2019-09-25T12:07:36.5140498Z 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]

R=me after tidy clean up and maybe slight rewording, but leave it to you to decide if it's worth it.

[estebank]

@bors r+

[bors]

📌 Commit ecfe92f has been approved by estebank