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

[David Spickett via llvm-commits]

DavidSpickett wrote:

QEMU instructions are here - https://linaro.atlassian.net/wiki/spaces/TCWGPUB/pages/29360783374/How+to+Reproduce+32+bit+Arm+Builds+Without+Hardware

I have not attempted to reproduce it that way yet, it may not especially with a gcc build. Try it if you like but no requirement to do so.

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