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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

> This breaks compilation with older macOS SDKs (Xcode 13 and older, with the macOS 12 SDK or older), with errors like these:
> 
> ```
> llvm-project/llvm/lib/CAS/OnDiskCommon.cpp:108:38: error: use of undeclared identifier 'F_ALLOCATEPERSIST'
>   FAlloc.fst_flags = F_ALLOCATEALL | F_ALLOCATEPERSIST;
>                                      ^
> 1 error generated.
> ```
> 
> And if building with a new enough SDK that contains this constant, how does it behave it attempting to execute it on an older version of macOS?

Let me fix that. This code path is not strictly needed for correctness. This code path only introduces safety margin when disk is almost out of place.

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