Decouple jsx element type from jsx factory return type and sfc return type

We need to look up the type of a jsx expression by actually resolving the jsx factory call, so that we don't create a reference to the global JSX.Element type, which can change shape between react versions (as it needs to in the react 16 upgrade). We also need to resolve the sfc return type and class element type from the parameters of the factory function overloads for the same reasons, doubly so because the types allowable as render method and SFC return values are no longer the same as JSX.Element (namely, they can be strings, arrays, portals, etc).

This might be considered a breaking change, because some consumers may expect JSX.Element to always be a supertype of both jsx element expression return types and SFC return types (even though this isn't true in react 16) - we certainly made that assumption internally, hence the need for the change. 🐱

Inizia tracciando il controllo delle espressioni JSX fino alla chiamata alla factory JSX risolta, quindi esamina come i parametri degli overload della factory forniscano i tipi degli elementi SFC e di classe. Il lavoro è completo quando questi tipi non dipendono più da JSX.Element globale per i casi React 16 indicati; nell'issue non sono indicati file o test.