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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

> > @rorth It seems that the `flock` is quite limited on Solaris but the check for `HAVE_FLOCK` is coming back true. It doesn't seem that it has a thread safe locking file locking mechanism on a mapped file from my initial research.
> 
> There's something very wrong here: Solaris has neither `flock` nor `LOCK_SH` anywhere in the system headers. No idea yet where this is from, though. Recompiling `llvm/lib/CAS/OnDiskCommon.cpp` with `-g3 -dD -save-temps` confirms this: `HAVE_FLOCK` is 0, as expected.

Ah, that makes sense. I guess `lockFileThreadSafe` just returns `std::errc::no_lock_available` that is actually `ENOLCK`. 

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