[no-undef] doesn't work for return types

[mjackson]

I can't get no-undef to warn me about missing types. I didn't see a no-undef rule in the @typescript-eslint rules, so I'd assume I could use the same no-undef rule that is present in core ESLint.

Repro

{
  "rules": {
    "no-undef": "error"
  }
}

export function fun(): ReturnType {
  console.log('wooo!');
}

Expected Result

A warning about ReturnType being undefined.

Actual Result

Nothing.

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	2.19.0
@typescript-eslint/parser	2.19.0
TypeScript	3.8.3
ESLint	6.8.0
node	12.13.0
npm	6.14.2

[bradzacher]

See #1856