[DiagnosticVerifier] Do not capture 'fatal error encountered' errors (#84640)
When the diagnostic verifier encounters a fatal error, it reports:
<unknown>:0: error: fatal error encountered while in -verify mode
If we capture this, and fail to match this against an expected-error,
the diagnostic verifier complains:
<unknown>:0: error: unexpected error produced: fatal error encountered while in -verify mode
<unknown>:0: error: diagnostic produced elsewhere: fatal error encountered while in -verify mode
The current workaround is to use -verify-ignore-unknown, but that in
turn may mask actual errors coming from unknown source locations.
Ignoring these errors about errors removes the need for that workaround.