ICE for a `use` statement inside of an `extern` block in `parse_foreign_mod_items()`

[kud1ing]

Example:

extern {
  use core::libc;
}

gives

$ RUST_LOG=rustc=1,::rt::backtrace rustc test.rs
rust: task failed at 'assertion failed: *self.token == token::RBRACE', 
rust/src/libsyntax/parse/parser.rs:3731
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further details and report the results to github.com/mozilla/rust/issues
rust: task failed at 'explicit failure', rust/src/librustc/rustc.rc:354
rust: domain main @0x7f99c880ba10 root task failed

[kud1ing]

Dupe of #6342.