Currently, the Rust buildbots (bors) all clone LLVM and other libs from Github. This seems to occasionally hang and cause mysterious spurious failures, possibly due to rate-limiting.

It could be better if we would do the git submodule update once and push the updated directory to the buildbots. We need to figure out how this interacts with lazy LLVM recompilation, but it should be possible.

cc @rust-lang/tools