Users shouldn't need 'noImplicitThis' on to benefit from 'ThisType<T>'

From #14141:

> * Otherwise, **if `--noImplicitThis` is enabled** and the containing object literal has a contextual type that includes a `ThisType<T>`, `this` has type `T`.

Is there a reason that users need `noImplicitThis` on to benefit from `ThisType<T>`? This seems unnecessarily restrictive, especially given that we want to give better support in JS files.