[llvm] [CAS] LLVMCAS implementation (PR #68448)

[Alexandre Ganea via llvm-commits]

aganea wrote:

Hello @cachemeifyoucan! Do you still plan on working on these PRs? There were some unaddressed comments/requests for changes left last year.

+1 with @dwblaikie - this would need to be split in smaller patches, there's just too much code to review. Can you send a PR for the `Path` changes? Can you send another PR for the basic/most indivisible interfaces with mock implementations in unit tests? I think you could then send specific implementations one by one. And finish with `llvm-cas` and the LLVM C APIs? My impression is that everyone agrees that we need all this.

https://github.com/llvm/llvm-project/pull/68448