WIP: Fix Appveyor CI build

[Rantanen]

Tasks

• Temporarily disable windows-gnu targets.
• Ensure stderr/stdout reference files use the expected line endings.
• Fix the line ending -related difference in clippy reports.

Summary

First of all, the windows-gnu targets cannot currently build compiler plugins rust-lang/rust#47029. These targets are disabled temporarily.

The big failure seems to be in how git checkout will convert the newlines in the stderr reference files into CRLF. This is countered by forcing LF line endings through .gitattributes.

There is still one final issue that I have yet to fix. For some reason the CRLF difference (I believe) affects the Clippy output in specific scenarios. This results in extra newlines in the stderr output.

A selected example from the needless_continue.rs below. The // [...] comments are added.

Windows, pasted Playground:

           if (zero!(i % 2) || nonzero!(i % 5)) && i % 3 != 0 {
               continue;
           }

           println!("Blabber");
           println!("Jabber");
           ...

Linux, gist Playground

   = help: Consider dropping the else clause, and moving out the code in the else block, like so:
           if (zero!(i % 2) || nonzero!(i % 5)) && i % 3 != 0 {
               continue;
           }
           println!("Blabber");
           println!("Jabber");
           ...

Note the issue can be replicated in playground when pasting code there in Windows. Looking at the JSON payload this does include \r\n linebreaks in the HTTP request.

I would love some pointers on where to look for that last issue.

---

The branch also builds on top of the #2333 to make it compile on the new Nightly to begin with.

[Rantanen]

Hm. Actually I've got no idea where this PR is going.

The test failures I'm receiving locally are not the same that AppVeyor reports. The windows-gnu failure is a valid fix, but the remaining ones Appveyor doesn't need.