Use new promoter to emit errors when required promotion fails

[ecstatic-morse]

This PR adds error reporting to promote_consts for when a #[rustc_args_required_const] argument cannot be promoted. This required some minor refactoring, notably TempStates are now determined in a separate MIR Visitor from the one that collects Candidates for promotion, and Candidates are validated at the same time they are collected.

This is another step on the road to removing the existing qualify_consts entirely.

r? @eddyb

[eddyb]

"context" doesn't feel right here. Maybe forces_explicit_promotion?

[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.

2019-10-29T18:27:51.8910920Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-10-29T18:27:51.9132076Z ##[command]git config gc.auto 0
2019-10-29T18:27:51.9193870Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-10-29T18:27:51.9254254Z ##[command]git config --get-all http.proxy
2019-10-29T18:27:51.9400893Z ##[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/65942/merge:refs/remotes/pull/65942/merge
---
2019-10-29T18:47:34.4166477Z    Compiling rustc_mir v0.0.0 (/checkout/src/librustc_mir)
2019-10-29T18:47:35.6672163Z error[E0425]: cannot find function `collect_temps_and_candidates` in this scope
2019-10-29T18:47:35.6672658Z     --> src/librustc_mir/transform/promote_consts.rs:1198:22
2019-10-29T18:47:35.6672952Z      |
2019-10-29T18:47:35.6674174Z 1198 |     let (temps, _) = collect_temps_and_candidates(tcx, body, &mut rpo);
2019-10-29T18:47:35.6674608Z      |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: a function with a similar name exists: `collect_temps_and_valid_candidates`
2019-10-29T18:47:41.5899547Z error[E0308]: mismatched types
2019-10-29T18:47:41.5899961Z     --> src/librustc_mir/transform/promote_consts.rs:1200:15
2019-10-29T18:47:41.5900260Z      |
2019-10-29T18:47:41.5900260Z      |
2019-10-29T18:47:41.5900832Z 1200 |         item: Item::new(tcx, mir_def_id, body),
2019-10-29T18:47:41.5901966Z      |               |
2019-10-29T18:47:41.5902402Z      |               expected reference, found struct `transform::check_consts::Item`
2019-10-29T18:47:41.5902402Z      |               expected reference, found struct `transform::check_consts::Item`
2019-10-29T18:47:41.5902798Z      |               help: consider borrowing here: `&Item::new(tcx, mir_def_id, body)`
2019-10-29T18:47:41.5903087Z      |
2019-10-29T18:47:41.5903436Z      = note: expected type `&transform::check_consts::Item<'_, '_>`
2019-10-29T18:47:41.5903774Z                 found type `transform::check_consts::Item<'_, '_>`
2019-10-29T18:47:45.8725207Z error: aborting due to 2 previous errors
2019-10-29T18:47:45.8725908Z 
2019-10-29T18:47:45.8726589Z Some errors have detailed explanations: E0308, E0425.
2019-10-29T18:47:45.8726928Z For more information about an error, try `rustc --explain E0308`.
---
2019-10-29T18:48:56.8905592Z   local time: Tue Oct 29 18:48:56 UTC 2019
2019-10-29T18:48:56.8905645Z   network time: Tue, 29 Oct 2019 18:48:56 GMT
2019-10-29T18:48:56.8905696Z == end clock drift check ==
2019-10-29T18:48:59.1575080Z 
2019-10-29T18:48:59.1688628Z ##[error]Bash exited with code '1'.
2019-10-29T18:48:59.1724341Z ##[section]Starting: Checkout
2019-10-29T18:48:59.1725905Z ==============================================================================
2019-10-29T18:48:59.1725955Z Task         : Get sources
2019-10-29T18:48:59.1726012Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

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