[llvm] [SandboxIR][Region][NFC] Change visibility of Region::add()/remove() (PR #129277)

[Jorge Gorbe Moya via llvm-commits]

https://github.com/slackito approved this pull request.

The Attorney stuff in the test looks a bit uglier now, but if it helps prevent accidental misuse I'm fine with it.

https://github.com/llvm/llvm-project/pull/129277