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

[Martin Storsjö via llvm-commits]

mstorsjo 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/114099