Support relative paths in VFSFromYAML

Use llvm::sys::fs::make_absolute to get an absolute path before
matching. Also, allow "." directories to enable testing.  ".." is still
not supported, and will require crossing file system boundaries to
implement correctly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202903 91177308-0d34-0410-b5e6-96231b3b80d8