What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No
• Not sure if this is considered a breaking change

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:
Look at this codeexample:

import { AsyncComponent, Component } from 'vue'
const a: AsyncComponent = () => ({
  component: new Promise<Component>((res, rej) => {
    res({})
  }),
})

const b: AsyncComponent = () => ({
  component: () =>
    new Promise<Component>((res, rej) => {
      res({})
    }),
})

The first example is like in the docs https://vuejs.org/v2/guide/components-dynamic-async.html#Handling-Loading-State and the second example should not work (I also looked at the implementation and I think this would not work).

The types are currently like such that the first example throws a type Error and the second throws no error. This PR fixes that.