Improves Clang's virtual file handling.

Improves Clang's virtual file handling.

This patch contains:

- making some of the existing comments more accurate in the presence
of virtual files/directories.

- renaming some private data members of FileManager to match their roles better.

- creating 'DirectorEntry's for the parent directories of virtual
files, such that we can tell whether two virtual files are from the
same directory.  This is useful for injecting virtual files whose
directories don't exist in the real file system.

- minor clean-ups and adding comments for class
FileManager::UniqueDirContainer and FileManager::UniqueFileContainer.

- adding statistics on virtual files to FileManager::PrintStats().

- adding unit tests to verify the existing and new behavior of FileManager.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125384 91177308-0d34-0410-b5e6-96231b3b80d8