[llvm] [CAS] Add MappedFileRegionArena (PR #114099)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

`fcntl(..., F_SETLK)` has some undesirable behavior comparing to `flock`, e.g., the processes need to be aware of not to open the same file twice, otherwise closing one file will lose the lock. Good that we identify the problem. I will have adding support to other BSD system as a future improvement.

https://github.com/llvm/llvm-project/pull/114099