rustc: Improve errors on private static methods

This gives a better NOTE error message when a privacy error is encountered with
a static method. Previously no note was emitted (due to lack of support), but
now a note is emitted indicating that the struct/enum itself is private.

Closes #13641
auto merge of #13647 : alexcrichton/rust/issue-13641, r=pcwalton

This gives a better NOTE error message when a privacy error is encountered with
a static method. Previously no note was emitted (due to lack of support), but
now a note is emitted indicating that the struct/enum itself is private.

Closes #13641