Fix Nuclide bug around tag @generated

Summary:
Cause:
Currently Nuclide will cache a file's status to readonly when the file contains 'generated' this cache is not cleared when user eliminates the string 'generated'. Thus, the file is always marked as readonly, even after user replaced 'generated', saved and reopen the file.

Fix:
On file save, clear the cache for generated tag.

Reviewed By: drmathochist

Differential Revision: D6776062

fbshipit-source-id: d9c238c15186372cae6c8a92f88747711740269e

pkg/nuclide-generated-files-rpc/lib/GeneratedFileService.js

@@ -53,6 +53,12 @@ export async function getGeneratedFileType(
   return tag;
 }
 
+export async function invalidateFileTypeCache(
+  filePath: NuclideUri,
+): Promise<void> {
+  cache.del(filePath);
+}
+
 export async function getGeneratedFileTypes(
   dirPath: NuclideUri,
 ): Promise<Map<NuclideUri, GeneratedFileType>> {