rustc: make comments on CrateContext doc comments
back: abi: remove dead code
back: archive: remove dead code
back: link: remove dead code
lib: llvm: remove dead code
metadata: common: remove dead code
metadata: csearch: remove dead code
metadata: cstore: remove dead code
doc: Remove dated "libgreen is default" statement.
metadata: decoder: remove dead code
metadata: filesearch: remove dead code
middle: astencode: remove dead code
middle: borrowck: remove dead code
middle: cfg: allow dead code
middle: const_eval: remove dead code
middle: dataflow: remove dead code
middle: freevars: remove dead code
middle: graph: ignore dead code
middle: kind: remove dead code
middle: lang_items: allow dead code