fix incorrect enum-value type names for enums used as inputs

luckily webapp is now a much more complete test suite and genqlient's
own tests, so it caught this!

generate/testdata/queries/InputEnum.graphql

@@ -0,0 +1,5 @@
+query InputEnumQuery($role: Role!) {
+  usersWithRole(role: $role) {
+    id
+  }
+}

generate/testdata/queries/InputEnum.graphql.go

@@ -0,0 +1,48 @@
+package test
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"github.com/Khan/genqlient/graphql"
+	"github.com/me/mypkg"
+)
+
+type InputEnumQueryResponse struct {
+	UsersWithRole []InputEnumQueryUsersWithRoleUser `json:"usersWithRole"`
+}
+
+type InputEnumQueryUsersWithRoleUser struct {
+	Id mypkg.ID `json:"id"`
+}
+
+type Role string
+
+const (
+	RoleStudent Role = "STUDENT"
+	RoleTeacher Role = "TEACHER"
+)
+
+func InputEnumQuery(
+	client graphql.Client,
+	role Role,
+) (*InputEnumQueryResponse, error) {
+	variables := map[string]interface{}{
+		"role": role,
+	}
+
+	var retval InputEnumQueryResponse
+	err := client.MakeRequest(
+		nil,
+		"InputEnumQuery",
+		`
+query InputEnumQuery ($role: Role!) {
+	usersWithRole(role: $role) {
+		id
+	}
+}
+`,
+		&retval,
+		variables,
+	)
+	return &retval, err
+}

generate/testdata/queries/InputEnum.graphql.json

@@ -0,0 +1,9 @@
+{
+  "operations": [
+    {
+      "operationName": "InputEnumQuery",
+      "query": "\nquery InputEnumQuery ($role: Role!) {\n\tusersWithRole(role: $role) {\n\t\tid\n\t}\n}\n",
+      "sourceLocation": "testdata/queries/InputEnum.graphql"
+    }
+  ]
+}

generate/testdata/queries/schema.graphql

@@ -62,6 +62,7 @@ type Topic implements Content {
 
 type Query {
   user(query: UserQueryInput): User
+  usersWithRole(role: Role!): [User!]!
   root: Topic!
   randomLeaf: LeafContent!
   convert(dt: DateTime!, tz: String): DateTime!

generate/types.go

@@ -372,7 +372,7 @@ func (builder *typeBuilder) writeTypedef(
 		builder.WriteString("const (\n")
 		for _, val := range typedef.EnumValues {
 			fmt.Fprintf(builder, "%s %s = \"%s\"\n",
-				typeNamePrefix+goConstName(val.Name),
+				typeName+goConstName(val.Name),
 				typeName, val.Name)
 		}
 		builder.WriteString(")\n")