It's possible for a header to be a symlink to another header. In this
case both will be represented by clang::FileEntry with the same UID and
they'll use the same clang::HeaderFileInfo.
If you include both headers and use some single-inclusion mechanism
like a header guard or #import, one header will get a FileChanged
callback, and another FileSkipped.
So that we get an accurate dependency file, we therefore need to also
implement the FileSkipped callback in dependency scanning.
Patch by Pete Cooper.