rustc: Fix a borrow failure in `.ln_str()`

`ln_str` caused ICE if `RUST_LOG=rustc::middle::liveness` is given
due to borrow failure.
auto merge of #11175 : klutzy/rust/ice, r=alexcrichton

`ln_str` caused ICE if `RUST_LOG=rustc::middle::liveness` is given
due to borrow failure.