message: Retry installing llvm on Kokoro Windows if failure
diff: if defined KOKORO_BUILD_ID (
  choco install llvm --version 6.0.1 -y
  +
  + :: retry if choco install failed
  + if errorlevel 1 (
  + choco install llvm --version 6.0.1 -y
  if errorlevel 1 exit /b %errorlevel%
  + )
  :: refreshenv is necessary to put LLVM on path
  refreshenv
  