[llvm-dwp] Include the debug_line.dwo section

This probably shouldn't be generated in the .dwo file for CUs, only for
TUs, but it's in the sample .dwos (generated by clang) so dwp should
reflect that.

Arguably the DWP tool could be smart enough to know that the CUs
shouldn't need a debug_line.dwo section and skip that even when it's
legitimately generated for TUs, but that's a bit more off-book.

llvm-svn: 254767