This adds the -verify-ignore-unrelated flag. When -verify is used without -verify-ignore-unrelated, diagnostics emitted in buffers other than the main file and those passed with -verify-additional-file (except diagnostics emitted at <unknown>:0) will now result in an error. They were previously ignored. The old behaviour is still available as opt-in using -verify-ignore-unrelated, but by being strict by default it should make it harder to accidentally miss diagnostics.
To avoid unnecessary performance overhead, -verify-additional-file is still required to parse the expected-* directives in files other than the main file.
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