[PATCH] Add a new pass to compute DWARF path discriminators.

[Eric Christopher]

  Seems like a reasonable start. I think it'd be good to start looking at breaking things out. The pass and then everything else (though some of the everything else can be split out into separate patches as well). Otherwise tests and documentation changes for the individual patches and it's probably good to go.

http://llvm-reviews.chandlerc.com/D2691