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

[Matt Arsenault via cfe-commits]

arsenm wrote:

Next step is to avoid all instances of "not --crash", which #128495 has a big chunk of 

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