Auto merge of #51866 - zackmdavis:hir_making_each_day_of_the_year, r=petrochenkov

add modifier keyword spans to hir::Visibility; improve unreachable-pub, private-no-mangle lint suggestions

#50455 pointed out that the unreachable-pub suggestion for brace-grouped `use`s was bogus; #50476 partially ameliorated this by marking the suggestion as `Applicability::MaybeIncorrect`, but this is the actual fix.

Meanwhile, another application of having spans available in `hir::Visibility` is found in the private-no-mangle lints, where we can now issue a suggestion to use `pub` if the item has a more restricted visibility marker (this seems much less likely to come up in practice than not having any visibility keyword at all, but thoroughness is a virtue). While we're there, we can also add a helpful note if the item does have a `pub` (but triggered the lint presumably because enclosing modules were private).

![hir_vis](https://user-images.githubusercontent.com/1076988/42018064-ca830290-7a65-11e8-9c4c-48bc846f861f.png)

r? @nrc
cc @Manishearth

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