Merge extendType functionality changes from #1322 (#1373)

* Extend the extend functionality

* Fix flowtype

* Extract logic from _makeInputValues to its own function

* Fix lint and minor refactors

* Fix type error due to copy & paste

* Fix array syntax

* Fix test more verbose

* Removed unnecessary check

* Extend directive

* Moved new tests into existing tests

* Fix lint errors

* Added missing description

* fix enum value error test

Author: mjmahone

src/type/definition.js

@@ -989,6 +989,7 @@ export class GraphQLUnionType {
   name: string;
   description: ?string;
   astNode: ?UnionTypeDefinitionNode;
+  extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>;
   resolveType: ?GraphQLTypeResolver<*, *>;
 
   _typeConfig: GraphQLUnionTypeConfig<*, *>;
@@ -1081,6 +1082,7 @@ export class GraphQLEnumType /* <T> */ {
   name: string;
   description: ?string;
   astNode: ?EnumTypeDefinitionNode;
+  extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>;
 
   _values: Array<GraphQLEnumValue /* <T> */>;
   _valueLookup: Map<any /* T */, GraphQLEnumValue>;
@@ -1230,6 +1232,7 @@ export class GraphQLInputObjectType {
   name: string;
   description: ?string;
   astNode: ?InputObjectTypeDefinitionNode;
+  extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>;
 
   _typeConfig: GraphQLInputObjectTypeConfig;
   _fields: GraphQLInputFieldMap;