While working with the NodeBuilder methods that are exposed from TypeChecker, I noticed that they might return undefined, but the return type doesn't include undefined.
This changes all return types to an undefined union.

I don't know if this is the right thing to do. Shouldn't these methods throw an error if the input does not match the NodeBuilderFlags? With the current implementation you cannot tell what's wrong, you just get undefined.

In addition signatureToSignatureDeclaration can return a node with typeArguments if you pass NodeBuilderFlags.WriteTypeArgumentsOfSignature. This method now returns an IntersectionType because there's not way to extend SignatureDeclaration in another interface.
This could be changed to return an interface that extends SignatureDeclarationBase instead.