Expose that the unique file ID has a device and a file component.

The use of sd_dev and st_ino has reached libclang, so expose the two components
in UniqueID so that we can use it in clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187616 91177308-0d34-0410-b5e6-96231b3b80d8