de-orphan extended information
Bizarrely, librustc_passes, librustc_plugin, librustc_mir, and libsyntax [weren't getting their error explanations registered](https://github.com/rust-lang/rust/issues/35284) (leaving _several_ error codes absent from [the index](https://doc.rust-lang.org/nightly/error-index.html) and `--explain`). This surfaced a few latent doctest failures that were fixed where readily possible and ignored (with a recorded excuse) if not.
Also, we don't issue E0563 anymore.
r? @GuillaumeGomez
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