[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?

https://github.com/llvm/llvm-project/pull/159133 should address that.

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