rustc: Fix a call to with_path in trans_unboxing_shim

This was causing lots of ICEs in cargo. I sadly wasn't ever able to reduce the
test case down, but I presume that's because it has to do with node id
collisions which are pretty difficult to turn up...
auto merge of #15370 : alexcrichton/rust/fix-cargo, r=pnkfelix

This was causing lots of ICEs in cargo. I sadly wasn't ever able to reduce the
test case down, but I presume that's because it has to do with node id
collisions which are pretty difficult to turn up...