Extend the dumping infrastructure to deal with additional
sections for debug info. These are some of the dwo sections from the
DWARF5 split debug info proposal. Update the fission-cu.ll testcase
to show what we should be able to dump more of now.

Work in progress: Ultimately the relocations will be gone for the
dwo section and the strings will be a different form (as well as
the rest of the sections will be included).