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

[Akshat Oke via cfe-commits]

optimisan wrote:

Perhaps ProgrammersManual should also be updated? Currently it says this:

> In situations where you absolutely must emit a non-programmatic error and the Error model isn’t workable you can call report_fatal_error ... The use of report_fatal_error in this case is discouraged.

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