Track skipped files in dependency scanning.

Track skipped files in dependency scanning.

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.

Reviewers: bruno, pete

Reviewed By: bruno

Subscribers: cfe-commits, jkorous, vsapsai

Differential Revision: https://reviews.llvm.org/D30881

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331319 91177308-0d34-0410-b5e6-96231b3b80d8