Docker image: facebook-clang-plugins/clang/src/download/llvm missing please run src/prepare_clang_src.sh #1316

Seems some recent Clang plugins changes broke the build of the Docker image:

Please include the following information:

• The version of infer 5069075
• Debian Docker
• facebook-clang-plugins/clang/setup.sh in Docker
• The full output in a paste, for instance a gist.
• If possible, a minimal example to reproduce your problem (for instance, some code where
  infer reports incorrectly, together with the way you run infer to reproduce the incorrect
  report).

Notable, the src/prepare_clang_src.sh is not in the tree...