[clang] [llvm] [mlir] [ErrorHandling] Add reportFatalInternalError + reportFatalUsageError (NFC) (PR #138251)

[Alex Bradbury via cfe-commits]

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

LGTM, thank you! I agree it would be good to migrate more report_fatal_error instances, but that's best handled in follow-up PRs IMHO.

https://github.com/llvm/llvm-project/pull/138251