[PATCH] Remove group-parent references.

[Rui Ueyama]

Hi Bigcheese, shankarke, atanasyan,

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.

http://reviews.llvm.org/D3945

Files:
  docs/design.rst
  include/lld/Core/Reference.h
  lib/ReaderWriter/Reader.cpp
  lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
  test/core/gnulinkonce-rearrange-resolve.objtxt
  test/core/gnulinkonce-remaining-undef.objtxt
  test/core/gnulinkonce-resolve.objtxt
  test/core/gnulinkonce-simple.objtxt
  test/core/sectiongroup-gnulinkonce-error.objtxt
  test/core/sectiongroup-rearrange-resolve.objtxt
  test/core/sectiongroup-remaining-undef.objtxt
  test/core/sectiongroup-resolve.objtxt
  test/core/sectiongroup-simple.objtxt
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