[PATCH] add support for LayoutAfter/LayoutBefore references

[kledzik at apple.com]

  The before/after model can lead to weird impossible to support scenarios where A has be before B but B has to come after C.  We email discussed a while back an alternative to layout-before and layout-after.  The model is to have groups and layout-after.   Aliases are handled by being zero-size atoms with a layout-after reference to what they alias.  When everything in a section should be kept, the first atom is the group leader, there is a layout-after chain for all the others and all the others have a in-group reference back to the first atom.

http://llvm-reviews.chandlerc.com/D373