[libclang] Introduce clang_getFileUniqueID which returns a struct
for a CXFile containing device/inode/modification time.

Intended to be useful as a key associated with a unique file across
an indexing session.

rdar://13091837

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@173559 91177308-0d34-0410-b5e6-96231b3b80d8