[ErrorHandling] Add reportFatalInternalError + reportFatalUsageError (NFC) (#138251)
This implements the result of the discussion at:
https://discourse.llvm.org/t/rfc-report-fatal-error-and-the-default-value-of-gencrashdialog/73587
There are two different use cases for report_fatal_error, so replace it
with two functions reportFatalInternalError() and
reportFatalUsageError(). The former indicates a bug in LLVM and
generates a crash dialog. The latter does not. The names have been
suggested by rnk and people seemed to like them.
This replaces a lot of the usages that passed an explicit value for
GenCrashDiag. I did not bulk replace remaining report_fatal_error usage
-- they probably require case by case review for which function to use.