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

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

Please include the following information:
- [x] The version of infer https://github.com/facebook/infer/commit/506907568b93ae29c3ff74bcdfcdb09903dbe2db
- [x] Debian Docker
- [x] `facebook-clang-plugins/clang/setup.sh` in Docker
- [ ] The full output in a paste, for instance a [gist](https://gist.github.com/).
- [ ] 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...