Rewrite `box_region.rs` to use generator resume arguments

[jonas-schievink]

box_region.rs makes use of generators, and passes arguments into them via thread-local storage. Now that #68524 has landed, this can be rewritten to use generator resume arguments instead, which is much cleaner and possibly also faster.

[bjorn3]

I can't find any uses of box_region anymore: https://github.com/rust-lang/rust/search?q=box_region&unscoped_q=box_region

[jonas-schievink]

src/librustc_interface/passes.rs uses it via the exported macros