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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Feels like a code generation issue but I'm not sure of it. I tried various versions of clang and all of them had the same problem. Next step is to reduce the input down to either stumble upon a rouge +4, or get something I can reduce to IR and report a bug.

So leave this feature disabled for 32-bit and I will handle this problem for now.

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