[clang] fix: replace report_fatal_error with Diags and exit (PR #147959)

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woruyu wrote:

Hi, at hstk30-hw, @shafik, @AaronBallman just wanted to check if there’s anything further needed from my side for this patch. I’d be happy to help clarify or adjust anything.

https://github.com/llvm/llvm-project/pull/147959