Forward declare FileEntry and DirectoryEntry in Module.h

FileManager.h is an expensive header (~350ms for me in isolation), so
try to do without it.

Notably, we need to avoid checking the alignment of FileEntry, which
happens for DenseMap<FileEntry*> and PointerUnion<FileEntry*>. I
adjusted the code to avoid PointerUnion, and moved the DenseMap
insertion to the .cpp file.

Globally, this only saved about ~17 includes of the related headers
because SourceManager.h still includes FileManager.h, and it is more
popular than Module.h.