[llvm] [CAS] Add LLVMCAS library with InMemoryCAS implementation (PR #114096)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

@nikic thanks for reporting. I guess we were not yielding the thread so it dead-lock when the parallelism is only two. Maybe the simpler fix is to just call producer first since the concurrency behavior that is being tested wasn't guaranteed either way.

https://github.com/llvm/llvm-project/pull/114096