Introduce a DirectoryEntryRef that stores both a reference and an

accessed name to the directory entry

This commit introduces a parallel API that returns a DirectoryEntryRef
to the FileManager, similar to the parallel FileEntryRef API. All
uses will have to be update in follow-up patches. The immediate use of the new API in this
patch fixes the issue where a file manager was reused in clang-scan-deps,
but reported an different file path whenever a framework lookup was done through a symlink.

Differential Revision: https://reviews.llvm.org/D67026

llvm-svn: 370562