Add GetDirectiveLocation extension method

[Shane32]

Closes #395 with a non-breaking solution. Not quite ideal, but good enough; argument fields and input object fields now use derived classes, and an extension method to ASTNode provides the directive location.

Other non-breaking options:

1. Move the method into ASTNode and override it in each derived class
2. Return null instead of throwing in case of error

Breaking options:

1. Move the method into IHasDirectives
2. Make the GraphQLInputValueDefinition ctor internal
3. Remove support for parsing into an GraphQLInputValueDefinition class

[Shane32]

@gao-artur Let's include this in v8 also (parser v9). I don't think I have any other changes to do.