Add appveyor.yml config for a future Windows bot.
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: 756ef344af921d95d562d9e9f9389127a89a6314@300786 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx