addAncestorsAsVirtualDirs("<stdin>") quickly returns without doing work
because "<stdin>" has no parent_path.  This violates the expectation
that a subsequent call to getDirectoryFromFile("<stdin>") would succeed.
Instead, it fails because it uses the "." if the file has no path
component.

Fix this by keeping the behavior between addAncestorsAsVirtualDirs and
getDirectoryFromFile symmetric.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266089 91177308-0d34-0410-b5e6-96231b3b80d8