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

[Rainer Orth via llvm-commits]

rorth 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.

I guess it would be way better to either skip or xfail the affected tests in `llvm/unittests/CAS/ProgramTest.cpp` with an appropriate comment (lack of `flock`) rather than in the depths of cmake files where it's both easily overlooked and (at least right now) has no explanation whatsoever **why** it is disabled on Solaris.

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