Use a pointer to keep track of the skeleton unit for each normal unit

Use a pointer to keep track of the skeleton unit for each normal unit
and construct it up front. Add address ranges at the end and a helper
routine so that we're not needlessly using an indirction in the case
of split dwarf.

Update testcases according to the new ordering of attributes on
the compile unit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198196 91177308-0d34-0410-b5e6-96231b3b80d8