Use Kind.DIRECTIVE_DEFINITION constant rather than string literal.

This is just for stability.  I know we don't do it everywhere right now, but
we should!

This also removes the extra function, but I don't think we're really
getting too much from that now, or later.  The constant comparison should be
the ideal way of doing this, and TypeScript should be able to do the type
narrowing on its own.

packages/apollo-server-core/src/utils/isDirectiveDefined.ts

@@ -1,20 +1,16 @@
 import {
   DocumentNode,
-  DefinitionNode,
-  DirectiveDefinitionNode,
+  Kind,
 } from 'graphql/language';
 
-const isDirectiveNode = (
-  node: DefinitionNode,
-): node is DirectiveDefinitionNode => node.kind === 'DirectiveDefinition';
-
 export const isDirectiveDefined = (
   typeDefs: DocumentNode[],
   directiveName: string,
 ) =>
   typeDefs.some(typeDef =>
     typeDef.definitions.some(
       definition =>
-        isDirectiveNode(definition) && definition.name.value === directiveName,
+        definition.kind === Kind.DIRECTIVE_DEFINITION &&
+        definition.name.value === directiveName,
     ),
   );