Auto merge of #7185 - ehuss:targetinfo-error, r=alexcrichton

Clean up TargetInfo

- The main motivation here is to provide a better error message when collecting information from rustc fails (it now shows the command and the output).
- Remove `has_cfg_and_sysroot`. I think this dates back to when it was introduced in #2328, as a guard for older versions of rustc that did not know about `--print=cfg`. Unless I'm missing something, I don't think we need to retain this backwards compatibility.
- Add some documentation.
- Demote the rustc cache log messages to `debug` level. I haven't really seen any caching issues, so I don't think it needs to be info level.
- Some other misc cleanup (remove unused function, etc.).