- isValid: FileManager only ever returns valid FileEntries (see next point)

- construction from outside FileManager (both FileEntry and DirectoryEntry).
  It's not possible to create a useful FileEntry this way, there are no setters.
  This was only used in FileEntryTest, added a friend to enable this.
  A real constructor is cleaner but requires larger changes to FileManager.

Differential Revision: https://reviews.llvm.org/D123197