DwarfDebug: Walk skeletons during fission pubtypes/pubnames emission

This more accurately represents the actual walk - pubnames/pubtypes are
emitted into the .o, not the .dwo, and reference the skeletons not the
full units.

Use the newly established ID->index invariant to lookup the underlying
full unit to retrieve its public names and types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196601 91177308-0d34-0410-b5e6-96231b3b80d8