Remove group-parent references.

Previously section groups are doubly linked to their children.
That is, an atom representing a group has group-child references
to its group contents, and content atoms also have group-parent
references to the group atom. That relationship was invariant;
if X has a group-child edge to Y, Y must have a group-parent
edge to X.

However we were not using group-parent references at all. The
resolver only needs group-child edges.

This patch simplifies the section group by removing the unused
reverse edge. No functionality change intended.

Differential Revision: http://reviews.llvm.org/D3945