This simplifies construction and usage while making the data structure
smaller. It was a holdover from the days when we didn't have a separate
DebugLocList and all we had was a flat list of DebugLocEntries.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214933 91177308-0d34-0410-b5e6-96231b3b80d8