Ambient classes and functions (concrete or ambient) can merge in TypeScript 3.6

The following code snippets will be considered valid in TypeScript 3.6 to more-easily express the "callable class" pattern.

For all intents and purposes, `C` should be considered a hoisted declaration like any other function.

```ts
declare class C { }
declare function C() { }
```

```ts
declare class C { }
function C() { }
```

CC @wesleywigham @RyanCavanaugh