The path canonicalization functions on win32 are intended to
canonicalize absolute paths; those with prefixes.  In other words,
things start with drive letters (`C:\`), share names (`\\server\share`),
or other prefixes (`\\?\`).

This function removes leading `..` that occur after the prefix but
before the directory/file portion (eg, turning `C:\..\..\..\foo` into
`C:\foo`).  This translation is not appropriate for local paths.