DebugLocEntry: Actually merge the loc entry when returning true.

Seems we didn't have any test coverage for merging... awesome. So I
added some - but hit an llvm-objdump bug while I was there. I'm choosing
not to shave that yak right now.

Code review feedback/bug catch by Adrian Prantl in r205360.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205373 91177308-0d34-0410-b5e6-96231b3b80d8
Add a doxygen comment to DebugLocEntry::Merge.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205374 91177308-0d34-0410-b5e6-96231b3b80d8