[c#] methodFullName for extension method calls

[xavierpinho]

Extension methods are syntactic sugar that underneath expand to the resolved method call, e.g.

x = new MyClass();
x.DoStuff(); // is actually rewritten by Roslyn as MyClassExtensions.DoStuff(x);

class MyClass {}
static class MyClassExtensions {
  public static void DoStuff(this MyClass c) {}
}

See: Binding Extension Methods at Compile Time

This PR is only a first move towards "full support" (as much as it's feasible) and fixes the methodFullName properties of those calls. So, in x.DoStuff(), it's still a dynamic dispatch call with an argument x, but whose fullName is no longer MyClass.DoStuff():<unresolvedSignature> but rather MyClassExtensions.DoStuff:void(MyClass). I'd like to refactor astForInvocationExpression a little bit more before piling more stuff into it, as it's quite large as it stands already.