## Summary
Fixes a small scoping issue in `DiagnosticId::matches`
Note: I don't think we should use `lint:id` in mdtests just yet. I worry
that it could lead to many unnecessary churns if we decide **not** to
use `lint:<id>` as the format (e.g., `lint/id`).
The reason why users even see `lint:<rule>` is because the mdtest
framework uses the diagnostic infrastructure
Closes#14910
## Test Plan
Added tests
6e11086c98