diff --git a/lib/Basic/FileManager.cpp b/lib/Basic/FileManager.cpp
index 4ac8e4d..adc9705 100644
--- a/lib/Basic/FileManager.cpp
+++ b/lib/Basic/FileManager.cpp
@@ -380,6 +380,10 @@
   return &UFE;
 }
 
+void FileManager::forgetFile(StringRef Filename) {
+  SeenFileEntries.erase(Filename);
+}
+
 const FileEntry *
 FileManager::getVirtualFile(StringRef Filename, off_t Size,
                             time_t ModificationTime) {