Fix aranges and split dwarf by ensuring that the symbol and relocation

Fix aranges and split dwarf by ensuring that the symbol and relocation
back to the compile unit from the aranges section is to the skeleton
unit and not the one in the dwo.

Do this by adding a method to grab a forwarded on local sym and local
section by querying the skeleton if one exists and using that. Add
a few tests to verify the relocations are back to the correct section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198202 91177308-0d34-0410-b5e6-96231b3b80d8