DebugInfo: Move the reference to the CU from the location list entry to the list itself, since it is constant across an entire list.

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