DebugInfo: Simplify debug loc list handling by keeping separate lists

Rather than using a flat list with "empty" entries (ala the actual
on-disk format), keep separate lists for each variable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204680 91177308-0d34-0410-b5e6-96231b3b80d8