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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

Ideally, we need `fcntl(F_OFD_SETLK)` but that is not POSIX and I don't see it on Solaris. 

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