Add appveyor.yml config for a future Windows bot.

I'm currently trying to get a libc++ Windows builder working on appveyor.
This patch adds the configuration file needed to build libc++ on Appveyor.
Details of where the buildbot can be found will come later

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300786 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed