If more than a prefix is provided - e.g. --check-prefixes=CHECK,FOO - we don't report if (say) FOO is never used. This may lead to a gap in our test coverage.
This patch introduces a new option, --allow-unused-prefixes. It currently is set to true, keeping today's behavior. After we explicitly set it in tests where this behavior was actually intentional, we will switch it to false by default.