Avoid failure to canonicalize '..'.

Avoid failure to canonicalize '..'.

Also fix completely broken and untested code which was hiding the
primary bug. The !LLVM_ON_UNIX branch of the ifdef was actually a no-op.

I ran into this in the wild. It was causing failures in our SDK build.

Ideally we'd have a perfect llvm::sys::fs::canonical, but at least this
is a step in the right direction, and fixes an obviously broken case.
In some sense the test case I've added here is an integration test. We
should have these routines thoroughly unit tested in llvm::sys::fs.

llvm-svn: 243597