Mark unused @template type parameter

TypeScript Version: 3.0.0-dev.20180711

Code

/**
 * @template U
 * @typedef {number} T
 */

Expected behavior:

U marked unused.

Actual behavior:

No error.

We already give the diagnostic, I just didn't see it because it only covers U and not @template. The error span will incidentally be expanded in #25748 to support the codefix deleting @template.