Undo the removal of the FileName member

This member will be needed after llvm#130627 anyway.

Author: jansvoboda11

llvm/include/llvm/Support/LockFileManager.h

@@ -49,6 +49,7 @@ class LockFileManager {
   };
 
 private:
+  SmallString<128> FileName;
   SmallString<128> LockFileName;
   SmallString<128> UniqueLockFileName;