Add detection of dead struct fields
Remove the dead code identified by the new lint
Mark relevant structs with repr(C)
auto merge of #14696 : jakub-/rust/dead-struct-fields, r=alexcrichton

This uncovered some dead code, most notably in middle/liveness.rs, which I think suggests there must be something fishy with that part of the code.

The #[allow(dead_code)] annotations on some of the fields I am not super happy about but as I understand, marker type may disappear at some point.