DebugInfo: Ensure unit IDs (for non-skeletal units) match thein index in the list

This simplifies reasoning about the code and enables simple navigation
from a skeleton to its full unit. (currently there are no type unit
skeletons, so the skeleton list doesn't have the same ID == index
property)

Eventually we should get rid of this ID and just store the labels we
need as the IDs are allowing this code to create difficult to
manage/understand associations (loops over non-skeletal units are
implicitly referencing their skeletal units during pub* emission, for
example). It may be necessary to have some kind of skeleton->full unit
association and a more direct pointer or similar device would be
preferable than an index.

llvm-svn: 196600