Add unstable `-Zdefault-hidden-visibility` cmdline flag for `rustc`. The new flag has been described in the Major Change Proposal at https://github.com/rust-lang/compiler-team/issues/656
Add unstable `-Zdefault-hidden-visibility` cmdline flag for `rustc`. The new flag has been described in the Major Change Proposal at https://github.com/rust-lang/compiler-team/issues/656