[lit] Make symlinks in test paths work a different way

Use os.path.normpath instead of realpath to collapse '..' and '.' path
components. Use realpath when caching search results about a path for
good measure.

I considered rigging up a test involving symlinks for this, but I doubt
I can check a symlink into SVN. The test would have to conditionally
create a symlink at runtime if the host OS supports it. This sounds too
fragile and complicated to me to be worth it.

llvm-svn: 312254