[PECOFF] Fix bug that section grouping may lay out atoms in a wrong order.

We added layout edges to the head atoms in grouped sections. That was wrong,
because the head atom needs to be followed by the other atoms in the *same*
section, not by the other section contents. With this patch, layout edges are
added from tail atom, which is the last atom in a section, to head atom.

llvm-svn: 189573