Add a DIELocList class to handle pointers into the location list.
This enables us to figure out where in the debug_loc section our
locations are so that we can eventually hash them. It also helps
remove some special case code in emission. No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203018 91177308-0d34-0410-b5e6-96231b3b80d8