1. Add a pass to fold debug label instructions so a debug info client can detect

1. Add a pass to fold debug label instructions so a debug info client can detect
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