VirtualFileSystem: Fix false positives in YAMLVFSWriter::containedIn

Checking if a path starts with another path isn't sufficient for
determining if one is contained within the heirarchy of the other.
We need to ensure that the substring ends at a directory boundary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209250 91177308-0d34-0410-b5e6-96231b3b80d8