rustc_privacy: Audit VisiblePrivateTypesVisitor for correct processing of items in blocks

[petrochenkov]

Spawned from https://github.com/rust-lang/rust/pull/29291/files#r43682594

VisiblePrivateTypesVisitor assumes that blocks can't contain exported items, but it's not true, blocks can contain impls, and they can be exported.