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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Actually I checked a bunch of points along the way and the addresses appear to be ok. Compiling the same thing with the host gcc does not crash, but I don't know yet if it's codegen or some implementation defined decision making the difference.

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