Use the result of privacy for reachability

This fixes a bug in which the visibility rules were approximated by
reachability, but forgot to cover the case where a 'pub use' reexports a private
item. This fixes the commit by instead using the results of the privacy pass of
the compiler to create the initial working set of the reachability pass.

This may have the side effect of increasing the size of metadata, but it's
difficult to avoid for correctness purposes sadly.

Closes #9790
auto merge of #9791 : alexcrichton/rust/reachable, r=catamorphism

This fixes a bug in which the visibility rules were approximated by
reachability, but forgot to cover the case where a 'pub use' reexports a private
item. This fixes the commit by instead using the results of the privacy pass of
the compiler to create the initial working set of the reachability pass.

This may have the side effect of increasing the size of metadata, but it's
difficult to avoid for correctness purposes sadly.

Closes #9790