Install LLVM 8.0.1 on AppVeyor

Author: nox

appveyor.yml

@@ -49,6 +49,7 @@ environment:
 install:
   - ps: Start-FileDownload "http://servo-rust.s3.amazonaws.com/build/MozillaBuildSetup-2.2.0.exe"
   - ps: .\MozillaBuildSetup-2.2.0.exe /S | Out-Null
+  - appveyor-retry choco install llvm --version=8.0.1
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host=x86_64-pc-windows-msvc
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin