empty ranges.
2. Reorg how MachineDebugInfo maintains changes to debug labels.
3. Have dwarf writer use debug label info to simplify scopes and source line
coorespondence.
4. Revert the merging of compile units until I can get the bugs ironed out.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31507 91177308-0d34-0410-b5e6-96231b3b80d8