[PATCH] Do not use layout-before to layout atoms.

[kledzik at apple.com]

  It looks like buildPrecededByTable() will "fix" atom chains that are not doubly linked list.  That is, if something only has the backward links to it predecessor, the layout will still work.  So, assuming everything has both links, I can see removing buildPrecededByTable() shouldn't break anything.

  But first it would be good to know where this is going.  What is the better long term design?

  Also, I'm not sure this pass needs this complexity for what ELF needs.  The darwin linker needs the follow-on map, but only when using an "order file" (a file with a list of symbols which specifies how to re-order the output content).  For ELF layout, sorting atoms by file ordinal then atom-ordinal-within-input-file (e.g. address order), will ensure the atoms in a section are kept as a cluster.

http://llvm-reviews.chandlerc.com/D3164