bug#63867: 29.0.91; typescript-ts-mode does not highlight function signature overloads

[Jimmy Yuen Ho Wong]

Reproduction:

1. C-x C-f test.ts
2. Insert:

```typescript
function fn():void;
function fn(arg1?:number, arg2?:string):void {
}
```

3. Observe the fn in the overload signature isn't highlighted as a
function name

Expectation:

Function signature overloads should highlight the function names.