[PATCH] Move DWARFUnitSection from inheriting SmallVector to holding an instance.

[Frederic Riss]

As it is explained in the commit log, this patch only moves from inheritance to composition, and the goal is to design a new interface better suited to DWARFUnitSection. This step is arguably what I should have posted first, the inheritance step wasn't really necessary. Now I could modify a lot more things in one go, but I usually get asked to split functional steps more not less :-)

http://reviews.llvm.org/D5352