[VirtualFileSystem] Support creating directories then adding files inside

Summary:
In https://reviews.llvm.org/D39572 , I added support for specifying
`Type` when invoking `InMemoryFileSystem::addFile()`.

However, I didn't account for the fact that when `Type` is
`directory_file`, we need to construct an `InMemoryDirectory`, not an
`InMemoryFile`, or else clients cannot create files inside that
directory.

This diff fixes the bug and adds a test.

Test Plan: New test added. Ran test with:

  % make -j12 check-clang-tools

Reviewers: bkramer, hokein

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40140

llvm-svn: 318445