Auto merge of #3198 - matklad:kill-command-type, r=alexcrichton

Remove CommandType struct

This removes `CommandType` struct as well as `cargo_rustc::process` function. So now all process creation goes thorough methods of `Compilation`.

This does change search path order from `util::dylib_path(), host_dylib_path()` to `host_dylib_path(), util::dylib_path()`, but I hope this is not a problem.

This also uncovers the fact that `rustdoc` is run sometimes with and sometimes without `host_dylib_path`. Is this intentional?