add x-ms-enum (Azure#17225)

Author: ArcturusZhang

specification/alertsmanagement/resource-manager/Microsoft.AlertsManagement/preview/2019-05-05-preview/AlertsManagement.json

@@ -650,7 +650,11 @@
       "enum": [
         "asc",
         "desc"
-      ]
+      ],
+      "x-ms-enum": {
+        "name": "SortOrder",
+        "modelAsString": true
+      }
     },
     "select": {
       "description": "This filter allows to selection of the fields(comma separated) which would  be part of the essential section. This would allow to project only the  required fields rather than getting entire content.  Default is to fetch all the fields in the essentials section.",

specification/alertsmanagement/resource-manager/Microsoft.AlertsManagement/preview/2019-05-05-preview/SmartGroups.json

@@ -437,7 +437,11 @@
       "enum": [
         "asc",
         "desc"
-      ]
+      ],
+      "x-ms-enum": {
+        "name": "SortOrder",
+        "modelAsString": true
+      }
     }
   },
   "definitions": {