[llvm] [SandboxIR][Region] Replace exit() with reportFatalUsageError() (PR #182134)

[Jorge Gorbe Moya via llvm-commits]

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

Looks good. The premerge test failure looks unrelated to me, please verify.

https://github.com/llvm/llvm-project/pull/182134