This means that JSDoc types can include the full range of Typescript
types now. It also means that Typescript annotations can include JSDoc
types. This is disallowed with a new error, however. But Typescript can
still give the correct types to JSDoc that shows up in .ts files by
mistake. This can easily happen, for example with types like

```ts
var x: number? = null;
var y: ?string = null;
var z: function(string,string): string = (s,t) => s + t;

// less likely to show up, but still understood.
var ka: ? = 1;
```

In the future, I will add a quick fix to convert these into the correct
types.

Fixes #16550

1. Remove checkJSDocFunctionType in favour of checkSignature.
2. Check that 'new', in addition to 'this', must be the first parameter.
3. Remove prematurely added JSDoc-quickfix test.

Also some cleanup from PR comments