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

[Rainer Orth via llvm-commits]

rorth wrote:

This patch breaks both the [Solaris/sparcv9](https://lab.llvm.org/buildbot/#/builders/13/builds/9575) and [Solaris/amd64](https://lab.llvm.org/staging/#/builders/120/builds/12336) buildbots.

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