Introduce the DWARFUnitSection abstraction.

A DWARFUnitSection is the collection of Units that have been extracted from
the same debug section.

By embeding a reference to their DWARFUnitSection in each unit, the DIEs
will be able to resolve inter-unit references by interrogating their Unit's
DWARFUnitSection.

This is a minimal patch where the DWARFUnitSection is-a SmallVector of Units,
thus exposing exactly the same interface as before. Followup-up patches might
change from inheritance to composition in order to expose only the wanted
DWARFUnitSection abstraction.

    Differential Revision: http://reviews.llvm.org/D5310

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217747 91177308-0d34-0410-b5e6-96231b3b80d8