DebugInfo: Omit DW_AT_addr_base from skeletal type units.

Type units have no addresses, so there's no need for DW_AT_addr_base.
This removes another relocation from every skeletal type unit and brings
LLVM's skeletal type units in line with GCC's (containing only
GNU_dwo_name (strp), comp_dir (strp), and GNU_pubnames (flag_present)).

Cary's got some ideas about using str_index in the .o file to reduce
those last two relocations (well, replace two relocations with one
relocation (pointing to the string index) and two indicies)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204506 91177308-0d34-0410-b5e6-96231b3b80d8