Frontend: Canonicalize to native paths when dumping module dependencies

Mixed path separators (ie, both / and \\) can mess up the sort order
of the VFS map when dumping module dependencies, as was recently
exposed by r224055 and papered over in r224145. Instead, we should
simply use native paths for consistency.

This also adds a TODO to add handling of .. in paths. There was some
code for this before r224055, but it was untested and probably broken.

llvm-svn: 224164