DebugInfo: Do not use DW_FORM_GNU_addr_index in skeleton CUs, GDB 7.8 errors on this.

It's probably not a huge deal to not do this - if we could, maybe the
address could be reused by a subprogram low_pc and avoid an extra
relocation, but it's just one per CU at best.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217338 91177308-0d34-0410-b5e6-96231b3b80d8