[explicit-function-return-type] allowTypedFunctionExpressions does not work for parameters

[davidje13]

Repro

{
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "sourceType": "module"
  },
  "plugins": ["@typescript-eslint"],
  "rules": {
    "@typescript-eslint/explicit-function-return-type": ["error", {
      "allowTypedFunctionExpressions": true,
    }]
  }
}

class Accumulator {
  private count: number = 0;

  public accumulate(fn: () => number): void {
    this.count += fn();
  }
}

new Accumulator().accumulate(() => 1);

Expected Result

No warnings; type of () => 1 is inferred from accumulate method signature, due to use of allowTypedFunctionExpressions

Actual Result

error: Missing return type on function (@typescript-eslint/explicit-function-return-type) at foobar/Accumulator.ts:9:30:
   7 | }
   8 | 
>  9 | new Accumulator().accumulate(() => 1);
     |                              ^
  10 | 

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	1.11.0
@typescript-eslint/parser	1.11.0
TypeScript	3.5.2
ESLint	5.16.0
node	12.4.0
npm	6.9.0

[bradzacher]

Apparently this wasn't ever added to the implementation. I've added handling for it in #680