Don't warn on fields in the `unreachable_pub` lint
This PR restrict the `unreachable_pub` lint by not linting on `pub` fields of `pub(restricted)` structs and unions. This is done because that can quickly clutter the code for an uncertain value, in particular since the "real" visibility is defined by the parent (the struct it-self).
This is meant to address one of the last concern of the `unreachable_pub` lint.
r? ``@petrochenkov``
6a42df7517