Generic solution for Implicit Conversions to Interval<T>

The author of https://github.com/google/cql/commit/def266a21cf8fb466434fb226b8a1c4ff7d9f8be did a one-off patch to fix the issue where we were unable to convert FHIR.Period to Interval, even though the modelInfo dictates this conversion. I just wanted to connect the dots here to the underlying more generic issue, which I had identified earlier in `convertGeneric` and is documented here: https://github.com/google/cql/blob/f4bb613e7ca5324e91bede1ab0eb4ebaa93cd6d5/internal/convert/convert.go#L529-L534

Since https://github.com/google/cql/commit/def266a21cf8fb466434fb226b8a1c4ff7d9f8be didn't mention the b/333923412 bug from there, I just wanted to connect the dots here between that change and the underlying more generic issue to make sure it doesn't slip through the cracks!