rustc: Don't allow priv use to shadow pub use

Previously, a private use statement would shadow a public use statement, all of
a sudden publicly exporting the privately used item. The correct behavior here
is to only shadow the use for the module in question, but for now it just
reverts the entire name to private so the pub use doesn't have much effect.

The behavior isn't exactly what we want, but this no longer has backwards
compatibility hazards.