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. 🐱

Start by tracing JSX expression checking to the resolved JSX factory call, then inspect how factory overload parameters provide SFC and class element types. Done means these types no longer rely on global JSX.Element for the stated React 16 cases; no file or test is named in the issue.