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

[Nikita Popov via llvm-commits]

nikic wrote:

Okay, it looks like the test hangs whenever only two cores or less are available. Trivially reproducible using `taskset -c 0,1 build/unittests/CAS/CASTests`.

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