Fix a buglet in remove_dots().

The function promises to canonicalize the path, but neglected to do so
for the root component.

For example, calling remove_dots("/tmp/foo.c", Style::windows_backslash)
resulted in "/tmp\foo.c". Now it produces "\tmp\foo.c".

Also fix FIXME in the corresponding test.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D126412