DwarfDebug: Simplify debug_loc merging

No functional change intended.

Merging up-front rather than delaying this task until later. This just
seems simpler and more efficient (avoiding growing the debug loc list
only to have to skip over those post-merged entries, etc).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204679 91177308-0d34-0410-b5e6-96231b3b80d8