[unified-signatures] false positive on async functions

[mysticatea]

Repro

{
  "rules": {
    "@typescript-eslint/unified-signatures": "error"
  }
}

interface Table<T> {
    getById(ids: ReadonlyArray<string>): Promise<Map<string, T>>
    getById(id: string): Promise<T | undefined>
}

Expected Result

No errors.

Actual Result

3:13  error  These overloads can be combined into one signature taking `ReadonlyArray<string> | string`  @typescript-eslint/unified-signatures

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	1.7.0
@typescript-eslint/parser	1.7.0
TypeScript	3.4.3
ESLint	5.16.0
node	10.15.3
npm	6.9.0

[bradzacher]

This was fixed in #839