[llvm-dwp] Correctly update debug_str_offsets.dwo when linking dwo files

[llvm-dwp] Correctly update debug_str_offsets.dwo when linking dwo files

This doesn't deduplicate strings in the debug_str section, nor does it
properly wire up the index so that debug_info can /find/ these strings,
but it does correct the str_offsets specifically.

Follow up patches to address those related/next issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254431 91177308-0d34-0410-b5e6-96231b3b80d8