[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.
> 
> 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.

The CMake check has updated to checking `flock` in [00dc72f](https://github.com/llvm/llvm-project/commit/00dc72fa1f328ae52554c5d404460de7381cb7f7). I don't think there is a good place to document this in the test as more and more tests will be added with `LLVM_ENABLE_ONDISK_CAS` requirement as more upstreaming happens. For now, I think it is quite natural to search for the configuration for this CMake option and found the comments in CMake.

I definitely want to create a centralized document for FIXME and TODO for the CAS implementations. Maybe a GitHub issue or just added in the doc page: https://llvm.org/docs/ContentAddressableStorage.html (I don't know if there are precedents for doing that).

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