Check symbols creation for C++

[narnaud]

Make sure the symbols created by symbols() make sense, particularly that the types are good.
I think at least the members have an empty type.

[LeonMatthesKDAB]

So at the moment the symbols themselves are of kind Field.
This just resolves to an instance of Symbol, which only knows about things like range and selectionRange.

And adding a general type field to that probably doesn't make sense.
As not all Symbols have a type (e.g. what "type" does a "Constructor", or an "Operator" have?)

But maybe adding a subclass "TypedSymbol" makes sense, which could know about the type.
I'll try adding that.