auto merge of #6251 : thestinger/rust/non_owned, r=pcwalton

Also fixed the docstring on `TC_ONCE_CLOSURE` (was accidentally the same as `TC_MUTABLE`) and shifted the `TC_EMPTY_ENUM` bit left by one since whatever previously used that bit has been removed.

diff --cc src/libcore/cell.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge