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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

To close the loop here, I start to write up some issues to track potential improvements and other tasks: https://github.com/llvm/llvm-project/issues/165324

WIP, and will also improve CAS documentation once the next few patches landed (so we have all the basic functions ready).

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