Properly display narrowed types when hovering in IDE

[ahejlsberg]

This PR corrects the type information displayed when hovering over variables in the IDE. Specifically, the getTypeOfSymbolAtLocation function used by the language service now consistently returns the narrowed type of a variable unless the variable is the target of an assignment. The PR also removes the old (and shimmed) getNarrowedTypeOfReference function and replaces all usages with calls to the new getFlowTypeOfReference function.

[mhegazy]

👍
can you also add a test for this, something along the lines of https://github.com/Microsoft/TypeScript/blob/master/tests/cases/fourslash/quickInfoOnGenericClass.ts