Add compiler docs testing to CI.
Fixes#47025.
I don't know if `x86_64-gnu` is the right builder for this, but there seems to be time left on [Travis](https://travis-ci.org/rust-lang/rust/jobs/307488864).
Remaining problems blocking this PR:
- [x] broken links caused by rustdoc issues:
- [x] `pub use self::Enum::...`: #46766 and #46767 (fixed by #47050, thanks @ollie27!)
- [x] `impl Deref for DerefToStdType`: #32129 (ignored in linkchecker)
- [x] `#[feature(decl_macro)]` and `use std::vec`: #47038 (ignored in linkchecker)
- [x] `rustc_data_structures::sync::{Lrc, RwLock}` aliases `std` types: #32130 (ignored in linkchecker)
- [x] markdown differences, in rust repository and in external crates, now failing the build with #46880 merged (all fixed)
- [x] multiple crate updates needed: `rand`, `log`, `parking_lot_core`, `flate2`
- [x] submodule updates needed to deduplicate dependencies: `rust-installer`, ~`cargo`~ (done by #47052)
- [x] #44953 test broken by `log` update (removed, this can be controversial)
- [x] Waiting `x86_64-gnu` build results ([done](https://travis-ci.org/rust-lang/rust/builds/323451069))
See individual commits for more details.
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