Debug info: Refactor DebugLocEntry's Merge function to make
buildLocationLists easier to read.

The previous implementation conflated the merging of individual pieces
and the merging of entire DebugLocEntries.

By splitting this functionality into two separate functions the intention
of the code should be clearer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215383 91177308-0d34-0410-b5e6-96231b3b80d8