Only infer 'never' in function expressions and arrow functions

[ahejlsberg]

This PR modifies the type inference rules such that we only infer a function result of never in function expressions and arrow functions. For regular functions and methods, in cases where we would have inferred never we now instead infer void. This matches the behavior we had before the introduction of never and ensures better backwards compatibility. For example:

class Base {
    overrideMe() {
        throw new Error("You forgot to override me!");
    }
}

class Derived {
    overrideMe() {
        // Code that actually returns here
    }
}

The above pattern is reasonably common and was broken because a never returning method cannot be overridden by a void returning method.

This PR only affects return type inference. It has no impact on functions and methods that explicitly specify a return type.

[mhegazy]

👍